Take to a shop. There are too many variables when troubleshooting solid state equipment. Could be anything from a transistor going out of tolerance to a resistor that may be out of tolerance or maybe a bad voltage for the right channel. That is assuming it is the amp. Have you tried taking the cable from the pre or the source and reversing right and left. What I mean is make the right channel output cable from the source and putting it in the left channel to the pre and left channel out to the right pre in? Then from the pre to the amp? If doing both and the problem stays with the right channel it is definitely the amp.
Good Luck
Good Luck