Quote: ….I’ve just never encountered another instance of a speaker posts not being grounded along with the rest of the amp.
I can’t find a reference ATM, but this is in fact a feature of the UFO amps. The “floating ground” allows bridging into mono without any loss of sound quality.
The other thing, that I didn’t notice the first time I looked at your picture, is it looks like you’re connecting the red/hot speaker wire to the RCA jack, not the black/ground one. That makes me think it is even more likely you are picking up noise.