First of all, everything you've heard about the sound quality of the Fosi Audio ZA3 is absolutely true and then some. I own an SMSL AO300 with which I've been very happy, and this thing EATS IT FOR BREAKFAST. Musicality, quickness, rhythmic drive, you name the category. And since it's bridgeable, it also dominates in handling tricky loads with more headroom. (Splurge and get the beefier power supply; it's worth it.)
Thing is, the designers were so fixated on minimizing the footprint -- presumably to hold the cost down and not just on pure aesthetics -- that they've left the consumer feeling like the fantastic sound came at a steeper price than monetary outlay alone would convey. For starters, I use relatively standard-sized speaker cables with relatively standard-sized banana plug terminations, and with them plugged into the Fosi correctly (together with the RCA-analog input cables), the subwoofer output is COMPLETELY inaccessible. I would have had to break my speaker cables to force enough room back there to connect a subwoofer. Then, when I tried to switch to cheap-ass bare wire speaker cable so that I could attach a subwoofer, I discovered that the speaker binding posts on the Fosi have threads that are perhaps 4mm in total length, so you can't loosen the nuts without them coming off and falling on the floor and rolling all over the place. Infuriating and completely unnecessary since the amp is already priced so comically low. If it was a $300 unit in a normal sized chassis with high quality connection terminals on the back, people would still be buying it in every bit as huge numbers as they do now; the demand for this kind of product isn't anywhere near as elastic as the folks at Fosi seem to think it is. Most of us have the extra money it would have taken to take this fantastic sound and put it in a much more ergonomically satisfying package.