Mine entirely shut down.....after about 5 minutes it worked fine.
I also had the "full volume rear speaker hiss" issue once. The car was garaged all night and OFF (stop switch hit twice). I merely opened my trunk and HISSSSSSSSSSSSSS....full volume rear speaker HISS. I then jumped in and started the car and could do nothing to shut off the sound. I finally ripped out the rear cover over the amp and unplugged the optical input, which killed it immediately. I could see a red light that was on inside the cable...then it flashed a few times, then went out. I plugged it back in and the sound did not come back on, and hasn't since. Strange. Although I do not think this is amp related, it is yet another premium audio gremlin.
Tim (apparently likes "3" cars)
E90 M3 -- Current ride
2004 GT3 -- Sold
1997 M3/4 -- Sold
1995 M3 -- Sold