Here's a mystery for you ...
Stereo Hi-Fi setup ... preamp, main amp, self-powered subwoofs.
Two line-level output jacks on preamp - one goes to subwoofs, the other to the NAD main amp.
If the main amp is turned OFF ... the subwoofs cut out too.
Pull inputs going to main amp, subwoofs work fine.
OLD amp - no subwoof interference.
Is there something about the line-level inputs on newer NAD amplifiers that "short"/bypass or damp any signal on the line when the amps are turned off ???
If so, any fix besides a buffer preamp to protect the signal from the main preamp - or yanking the plugs every time you want to test the subs ? A new pre-amp with more, isolated, line outputs ???
Any info appreciated.