My iFi Zen Phono has almost no audible background hiss with either MM or MC cartridges, but its SINAD was measured to be comparatively poor.
Same experience here... There are too many variables when dealing with these projects.
Real-world usage scenarios can differ from test bench measurements.
I own the Zen Phono and the MX Vynl, both ranked by ASR as somehow "worse" than the Fosi Box 5... For sure they seem to be, judging by the measurement results.
Anyway in the real usage the british stages here are silent even at the highest gain and their power adapters are not affected by any kind of mechanical hissing or buzzing.
The electrical system they’re connected to is the same, and so is the room...
Also, you raised an interesting point, and I regret not having tested with low-output cartridges.
I also own a ZA3 and no issues of any sort there, so reading through many conflicting comments, it seems plausible that the quality control for these early product batches isn’t yet top-notch, and the performance of individual units might vary.