The volume scale readout for movies to music isn't necessarily comparable either. Does sound like you''re asking a bit more out of the setup than the avr can handle. I'd also stop the bi-amping thing, just a waste of wire....but check your connections just in case you've got a stray strand of wire or something along those lines while you're at it....
ps You might think about a cooling fan for the avr....the avr has plenty of ventilation around it?
pps You do have a fan on a reread, as was asked what is it?