Originally Posted by
Retsek
These are a GA twin plug pair so they don’t have an Auto-on switch.
It's about 5 years since I looked at the switches in my A20 control modules but I can see no need for an Auto On switch with LEMO power. Mine simply turn on when avionics bus powers the LEMO jack. The on-line Owner's Guide shows the Auto Off switch so that may be what the article I linked earlier was referencing.
Does the hiss change if auto off is disabled?
I suspect your best bet would be to compare with other Bose A20. Perhaps a local flying club?