I bought an SP400 for my birthday to pair it with Soncoz SGD1 DAC, after testing the SP200.
-Headphones/IEM: HD800s (ASR EQd in Roon) via XLR, HD580(no EQ) via SE, Westone 4(no EQ) via SE.
-Transporter: Odroid c2 with Roon Bridge via usb-c into SONCOZ SGD1
-Interconnects: DIY XLR-XLR with Neutrik plugs and audionote cable.
Just my impressions as I have no way to measure equipment.
Coming from a Ray Samuels HR-2 Emmeline, I first tried the SP200 and I was really surprised how good it sounded compared to the HR-2.
SP200 to me sounded more detailed and open with all my headphones/IEM.
However, HR-2(RCA-RCA) is dead silent at max volume with IEM and SP200(RCA-RCA) was not, with noise being an issue.
Moving to XLR interconnects from the DAC, the SP200 was completely silent.
After about 1 week of using SP200, I started experienced some fatigue with HD800s.
Left/Right volume was a bit of hit and miss as well, where sometimes the right was much louder than the left channel.
Other times, the same song was volume balanced on both sides.
I tried to play with the Soncoz's volume(digital?) instead and try not to touch the volume on the sp200, for some time, but really I did not like this workaround as I wanted to keep the source at its max volume.
In the end, volume discrepancy, fatigue and also the amount of heat that the sp200 was generating in XLR,
were the main factors for me, to look at the sp400. (Topping A90 is not available on prime amazon.co.uk, albeit the pre90 is.)
SP400 to me sounded more relaxed then the SP200, and I did not experience fatigue.
That for me translated to a better user experience.
Left/Right balance volume is also perfect, to my ears.
The HD800s sounds bold and nice at high gain in between 45 and 60 volume (12/1 o'clock) for DSD tracks and in between 30 and 40 volume (10/11 o'clock) for non DSD tracks.
As I apply EQ to the headphone via Roon, without EQ those volumes are even lower with the HD580.
I think the SP400 is an amazing headphones amp, it sounds really good, but I also think that it is an incomplete product, unless my sample is broken.
I was really disappointed to find out that the pre-out has no control.
I am not sure how I can drive 2 mono blocks in the living room with the SP400, if I also want to listen to headphones during quiet times.
More care could have been put, in the design and implementation of the overall package:
-no remote and/or software control for the pre-out: pre-out cannot be disabled.
-no manual and/or mechanical control for the pre-out: headphone plugged in does not disable pre-out, neither there is a physical switch for such selection.
-no usb port in the device, and/or WIFI (like SONCOZ) to update the firmware.
-no firmware upgrade path, albeit there is a specific entry in the menu about VERSION, showing 1.0 for both HW and SW, I guess this stays as is, unless you buy a new unit.
I haven't heard any other headphone amplifier as clean as this, it is a great sounding amp, and perhaps just this, justifies the higher cost, but I am sure there are many other amps that can sound as good as the SP400 and can provide an overall better package.