Since most modern Hi-Fi amplifiers should sound the same when used within their performance limits, I am curious about your basis for making an audiophile-type claim of a "huge improvement"? Were you driving the Fosi amplifier into distortion with loud music and hard-to-drive loudspeakers? Did you have noise problems with the Fosi that the SMSL DA-9 that the balanced input fixed? (Unlike some other SMSL amplifiers, this one apparently has true differential balanced XLR inputs.)
The SMSL DA-9 was discussed here at ASR
[LINK] in a long (41 page) thread which I have not read in its entirety. That thread seems to contain a fair bit of sighted listening, subjectivist style "huge improvement" claims. That being said, it still might be a good budget amplifier, and it does seem to be developing a base of enthusiastic fans.
The DA-9 uses a built-in Meanwell power supply and not a wall wart or external brick-type PS, which I prefer. It is based on the new technology Gallium nitride on Silicon carbide transistors, but I do not know if the chips are the same as in excellent, but hard-to-find 40wpc Loxjie A30 that
@amirm measured. However, the manufacturer's measurements are apparently limited to rudimentary single frequency distortion ratings. Sold by Shenzhen for $250 on Amazon, it's a bit more costly than the $150-200 range of this thread, although not by much. I find it appealing, but I will reserve judgement until
@amirm puts one through its' paces. (I find it interesting that SMSL says that it
has "two levels power input filtering brings its own power supply purification process to thoroughly filter the mains interference" which supports recent statements by
@amirm regarding power conditioning.