That would make some sense to me as the amp is probably getting all of the signal from both sides going to the one speaker... and as such I'd check the amp. Move the RCA's to the other set (the other 2 channels).

One thing I would certainly check is it sounds like you could have your amp set to a mono feed (usually used for subs). IIRC, my old kenwood kac744 had 3 settings, a stereo setting, a L+R setting (each speaker plays everything) and a mono setting (usually used for subs).

Check that little switch on your amp and make sure you are messing with the right side of the amp, it's easy to get the sides mixed up with all the little knobs, switches and inputs.