What a crazy issue... I originally thought my WS420 had locked out my boat and tower amps(not subs) kinda how it dims them when you click the mic on... So, when it did this at the lake I wired around it... head unit right to amp... and it worked fine.
Forgot about it till this year and wanted the mic as it come in handy for teaching to the various folks on this huge trip. Got a new one, wired it in and everything worked fine...
Then at the lake, same thing... audio goes down, but subs still work... wierd... try to wire around it and the head unit won't pass audio either. When I get home I pull out all the seats and find the protect light is on on my amps (small boat, hard to mount things where you can see both ends of the amp) go through all kinds of things and finally discover my ground wire from the battery to my distro is corroded and this was my problem all along! It was the only thing not passing the continuity test... took it apart, cleaned it all off and resintalled... everything works again!
and now I have an extra WS420... to the classifieds!
moral of the story is maybe check everything next time before ordering new stuff.