Try routing the preamps L channel to the R input on the amp, and the R preamp output to the L input on the amp. If the problem switches speakers, then the problem is in the preamp or the source. If the problem does not switch speakers, then the problem actually lies in the amp rather than the preamp.
That's my recommendation before you assume it's a problem with the preamp.
Michael
That's my recommendation before you assume it's a problem with the preamp.
Michael