Yamaha rx 596 problem

This "mid fi" receiver's about 7 years old. All of a sudden, the left channel is trebly & tinny and has no bass. The right channel is bass heavy & muddy & has no treble. when the volume is turned all the way down, the tinny sound still comes out of the left channel.

Its not a speaker problem because its also happening with the "B" speakers in the other room. And it happens no matter what the source- CD. phono, radio. I checked all the connections in the back & everything's in fine.

I guess its probably shot but anyone got any ideas before I go & gget a new receiver?