If the amplifier has pre-out and power-amp-in connections and you can borrow a second amplifier or receiver which also has those connections you should be able to localize the problem to either the preamp or power amp section by pairing one portion of your amp with the other portion of the borrowed amp/receiver.

Having said that it sure sounds as if the problem is in the pre-amp section.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8