S.M.S.L. SP400 Review

I recently did the S.M.S.L. SH-9 review and I came out fairly impressed at the sound quality the little amp produced for its price. The only things that personally bothered me was the fact the SH-9 didn’t have any pre outs and that it wasn’t a truly balanced amp. 3 watts is a good chunk of power for an amp but I wanted that true balanced design. The SP400 fills in the gaps with the truly balanced design, an extra 3 watts(6W total) out of the balanced port and at least xlr pre outs. It also has a bigger power supply over the SH-9 and uses the very impressive stepped volume relay system that was in the SH-9 as well. On paper this seems like a decent upgrade over the smaller SH-9.


Gear used

S.M.S.L. SU-9, S.M.S.L. SH-9, Ikko OH10, DUNU LUNA/ZEN, ADV M5-5D, HEDD HEDDphone and Audeze LCD-GX.

Looks and Feel

The SP400 is rather big when compared to something like the SH-9 or iFi NEO iDSD. It isn’t any taller than the SH-9 but its longer and wider which means it takes up a little more space so this is something to think about for desk use. The top of the unit also has a nice glass top which looks great but picks up fingerprints like crazy. The unit itself isn’t very heavy which means It doesn’t stay still when plugging or unplugging headphones into it without holding the top of the device which will leave fingerprints. When I put the SU-9 on top of it, the SP400 stays down and makes swapping headphones a breeze without the device moving around. The display is the same used in both the SH-9 and SU-9 so it matches well with the SU-9 if you don’t wanna pick up the M400 DAC that matches the SP400. The placement of the single ended and balanced ports works out in my favor being closer to the middle of the device vs being far off on the end of the amp like the SH-9.

Accessories and unboxing

The unboxing is pretty straight forward. Inside the box you get the SP400, remote, power cable and the standard user guide and QC check card. The top has a plastic wrap attached to protect the glass from scratches during shipping. The SP400 also comes wrapped in a cloth-like bag. A Lot of effort went into making sure nothing gets scratched or broken during shipping.


The UI is nice but unless you use the remote, you have to do a lot of steps to do simple tasks that could easily be done if there were switches or buttons on the front of the SP400. Things like input select and gain select all have to be done via the UI unless the remote is used(which has dedicated buttons for both). Outside of the input and gain options, we also get a volume mode which allows the option(disabled) to have each volume change activate the relay or the option(enable) to allow you to move through the volume range and when you stop the relay will switch to the desired volume level. I find the enable option better when I swap from IEMs to full size headphones as the volume needed between the two is vastly different. This saves the relay life though I’m really not sure how long the relays are meant to last. The last two options are the brightness and Firmware version info. 

Sound (overall)

These final impressions were done off the S.M.S.L. SU-9 connected to the SP400. This will be what the SP400 sounded like with all the headphones I used. Since this is an amp, my sound section is mostly going over the common things I picked up from my headphones vs how they perform on other source gear. Things like DAC selection as well as different headphone pairings will produce different results and impressions vs what my ears hear on my specific gear.

The lows are very accurate sounding. Lows sound a little faster and even some of my slower IEMs felt a little spunky in terms of speed when attached to the SP400. I would call this still pretty neutral though. When music calls for low end thump and rumble, the SP400 definitely delivers. I found no issues with the low end and I was really impressed by the overall presentation down low with the SP400. The mids are super clear and I have no issues here as well. There’s a nice balance with vocals sounding neither too far or two close. That being said, headphones will mostly dictate whether mids like vocals are recessed or not. For headphones that have a mid range bump, the SP400 is a real treat. The highs are also really done on the SP400. The SP400 felt like it had better control over the highs. At first I thought it may be somewhat warm sounding as I didn’t get that clinical “metallic” sound signature I was used to with other well measuring amps. Some headphones that I know are super bright, felt slightly relaxed while still having great details up top. It feels like a less clinical top end and more enjoyable while maintaining fantastic detail retrieval.


While I find headphones to be in charge of width and depth, I did notice that pretty much all my headphones had a wider and deeper soundstage vs other things I was plugging them into. It’s not an extreme effect but It was easily noticeable to my ears. Imaging is fantastic as well and in combo with the soundstage, can really make for some wonderful listening sessions.

Inputs and outputs

You get the normal RCA and XLR inputs but the SP400 does have XLR outs. Why they skipped having an RCA out is strange to me. Looking at the back of the unit, there really is no room to stuff RCA outs and I’m gonna guess this is what it came down to. This was designed to stack with the M400 so some sacrifices had to be made I suppose.

Personal gripes with the SP400

While I don’t have any real “deal breaker” issues with the SP400, a few things do irritate me. When the volume knob rotates, it has a felt click. This is fine and one would assume that the “click” they feel would result in one volume change. For the unit I have, it requires two of these felt “clicks” in order to move the volume up or down on notch. I find this super weird and I can’t tell if this is a unit defect or if all units exhibit the same thing. It’s not a show stopper but more of a “WHY?” kind of thing for me. The top is also a fingerprint magnet. This is expected as it’s a black glass top. They didn’t treat it with any fingerprint coating so it can get real ugly fairly quick. While I’ll get into it below, the gain could use a little more output as well.  Lastly is the heat this unit produces. I’ve had this over a month with no issues but the SP400 gets fairly hot after about an hour of being on. Like class A hot. This is really strange as I’ve not used any other THX amps that get hot. The SH-9 got a little toasty but it wasn’t anything like this. You won’t burn your hands on the unit but it becomes uncomfortable after a few seconds touching it. I’ve left my phone on top of it a few times which resulted in my phone being super toasty when I picked it up. Those are pretty much my only gripes with the SP400 which in all honestly are mostly personal nitpicking.

Single ended/balanced power outputs and gain

The SP400 is a truly balanced amp so we end up getting 3 watts into 32 ohm via the single ended output and with the balanced output, we get 6 watts into 32 ohm. The power from the balanced port is enough for most headphones I can think of. There was no issue powering the HEDDphone or LCD2C. The biggest complaint I have however is with the gain options. The difference between all three gain options isn’t honestly much in terms of volume. As an example, I had the volume for a specific IEM at 31 for low gain, 27 for medium and 25 for high gain. Luckily I picked up no noise from the balanced jack on high gain so I left it on high gain and never messed with the lower gains. S.M.S.L. could have pushed medium and high gain to higher DB ranges. I had zero issues powering any headphone in high gain and the max volume I reached being 74/99 which was on the high side for me. For my personal tastes, I would have preferred to max out at something lower than 74 on high gain.

Over-ear pairings


The HEDDphone pairs decently enough with the SP400. The somewhat relaxed treble still sounds a little dull to my ears. The HEDDphones do sound a little wider and deeper here. Low end has a nice impactful sound when it’s called for. The low end is still a little too light for my personal taste and the Sp400 doesn’t do much in the way of giving it the little extra boost I’d prefer. Vocals still retain their magic with the HEDDphone and while I don’t mind this pairing, I would use these headphones with something a little warmer sounding such as the iFi iDSD Signature or something from Schiit.

Audeze LCD-GX

The GX doesn’t need much power at all to sound good. It does scale a little with better gear and while the SP400 didn’t drastically change the sound quality of the GX, it did benefit from the wider and deeper soundstage that the SP400 provided. Like with pretty much all the headphones I tested with the SP400, the GX sounded very well controlled and tight. Low end still has some wonderful impact and the top end had just a bit more energy overall. This was a rather enjoyable pairing.

Amps comparisons


The NEO is another fairly reference sounding DAC/amp and I found this to be an interesting comparison. The NEO has a fairly common sounding low end but it lacks the same impact and tightness the SP400 provides. The mids on the NEO have a slight bump that the SP400 is missing. This can make some vocals on headphones sound a little bright at times. The NEO also has the music mostly in your face vs the deeper sounding SP400, treble performance is mostly the same with the NEO having a slightly shaper top end. The SP400 ends up winning when it comes to soundstage and I think overall tuning for an amp. The NEO is still a great reference amp however and comes in much cheaper than the SU-9/SP400 stack.

S.M.S.L. SH-9

For the most part, the two amps share a lot of the same features and sound signature. While I really liked the way the SH-9 sounded, I started noticing big differences once I started A/B testing the two. The SH-9 struggles with low end impact on full size headphones. With IEMs it sounds fine however. Full size headphones start to sound a leaner the higher in volume you go. This is something the SP400 doesn’t suffer from. Both have about the same mids and highs performance with the SP400 sounding like it has better control up top. The SH-9 has a deep soundstage and average width while the SP400 has a wider stage but with the same deep stage of the SH-9. Both are really nice amps. The SP400 does have a XLR out so there is a little extra function with the SP400 as well. I think it will come down to long term plans for an amp. If you plan to buy something to use for a few years I’d say grab the SP400. If you wanna try stuff out and have multiple desktop amps, then the SH-9 will be easier on the wallet.

iFi micro iDSD Signature

The iDSD Signature is a slightly warmer sounding device vs most stuff I’ve tried. I really like the smooth and warm sound the iDSD Sig produces so I was curious how it does directly against the SP400. The bass impact is actually about the same between both devices with the lower mids sounding a bit warmer on the iDSD Sig. The higher frequency detail retrieval goes to the SP400. While both have a somewhat tame top end, The SP400 simply brought in more details. Soundstage was wider on the SP400 as well. I really like the iDSD Signature and if you need an all in one device and don’t want to squeeze every detail out of a desktop only setup then I’d say grab the Sig over the SP400.

Overall thoughts

The big question is gonna be whether or not the asking price of $630 is worth it for the SP400. To that I would have to say it depends on one's needs. If you need the XLR pre outs, want the relay volume control and want a good amount of power, then I do think the SP400 is worth the money. While I didn’t have a Topping A90 to test this against, reviews I’ve read seem to peg the A90 as a super neutral sound with little life in the sound. The SP400 is definitely full of energy and makes for a very exciting listen.

As such, I will be giving the SP400 a full recommendation as long as you have a need for the SP400’s extra features and better sound performance over something like the SH-9. I really had a blast with the SP400 and plan to keep it around for future reviews. I look forward to S.M.S.L.’s new VMV lineup coming out later this year. Thanks for reading!