By now I assume you have experimented enough and know that the audible noise, while very low, is audible with ears near the tweeter. Based on the specs and Gene's test results, this should not be the case, unless your room is really silent. Even if your room noise is in lower than say 15 dB (mine's about 25 dB and it seems quiet to me), you should not be able to hear white noise.
The Amp10's 91-92 dB SNR at 1 watt is certainly not SOTA, most of the Hypex and Purifi amps, even the low cost ones can do better, but again, 91-92 dB is at least comparable to most of the best measured AVRs, and even the recently measured two channel Yamaha R-N2000A, and should be silent to the ears.
It sounds like the dealer won't let you try a second unit without coming over to hear it himself, that's fair, but I would say unless he has really convincing reasons, you should insist on trying a second unit, unless this audible noise does not bother you at all. It is far better to be firm on this and then you can enjoy without having second thoughts down the road.