I’m using the Topping Pre90, and am liking the sound quite a bit. The Fosi, IMO, is a very clean, lean and agile pair of amps. They are not warm or euphoric in the least. I feel the same way about the Topping, clean and lean.
Combining the two has given me a rig that is capable of extracting a lot of the subtleties available in any given recording.
I got the Topping to go with the SMSL VMV class A amp, which I also feel is a great match, provided your speakers aren’t too demanding and uber low in efficiency. I actually used the 10 watt amp with many speakers in the 86 or so dbl range with absolutely no issues in my small acoustically treated listening room (13x14).
I’ve got to stop watching the reviews by Harley at Pearl Acoustics on YouTube. I got the SMSL because he compared it favorably to the Pass amp (and way less warm - the reason I was interested in it) and the Topping Pre90 also got pretty rave reviews by him. The best part was that they were both affordable to the common man’s finances.
The Fosi I got into in a roundabout way. I got one of their older amps that had bluetooth capabilities because I wanted something small, with bluetooth that I could connect to my TV and hide the amp behind it. I used that one for a few years.
I then came across one of their newer BT integrated amps, which I also liked. I got tthat one as I was thinking of having some small speakers connected to them in my listening room for some background music when I didn’t want to turn on my main rig (was all tubes then). Then I came across the V3 monos on that forum where they crowd fund (?) and they seemed pretty interesting
Once I got them, I saw a few reviews about them with the Sparkos on YouTube and decided to pick up four (two each for the XLR inputs). Kind of crazy that the op amps cost more than the monos, themselves.
Once the temp gets colder I’ll swap the Fosi’s out for probably both the pass gear and tubed gear for some warmth. But until then, I’m in no hurry to swap out the Topping/Fosi combo.
I’m convinced that if someone put the Fosi into a large, heavy, blinged out, audiophile-approved chassis they would be all the rave. Provided they were priced at over $10,000 because, as we all know, more money better sound.