Just move the speaker wires on the Pannys. You shouldn't notice a difference, all four amp channels are identical.
Panny mute shuts off signal stage, but doesn't silence the amps which are still amplifying at full gain to speakers. The hum is getting into the amp somehow, probably through ground from signal. If your digital in is your only connection, try optical instead of coax if possible, or use a transformer decoupler to break a ground loop.
You might find that the Behringer DAC is better than the Panny. You could also try the Jan Didden mod kit if you want to improve the sound of behringer. It is pretty rough as stock.
Are you getting a digital signal out of the Behringer post DSP? How?