I have a pair of Bose A20 but they have never had the batteries fitted. When powered by the LEMO connector I do not hear any hissing with avionics master on and engine at idle. Normally my headset volume is close to maximum and audio level is controlled at the audio sources.
If you don't receive any other answers I may try installing batteries in one A20 and run a check with the headset not connected to the aircraft.