I have had a quick peek at the schematic for your amp and I can tell you that you are in for a really wild ride with this...
You should check the voltages around IC701 and IC702 and compare them to each other, with the same signal being applied to the amp. You will also need to check the voltages around IC501. If any look suspicious, you will need to check the limiter and booster circuits and their associated feedback paths. If they are all good, you will need to delve further back into the preamp, processor, selector and associated circuits. You have not been specific about which channel is faulty, or on which settings the fault presents itself etc so it's rather hard to help you.
A word of caution, the amps are direct coupled, so one slip of your meter lead shorting something will destroy components instantly and in a cascading manner.
If little of what I have mentioned here makes sense to you, my best advice to you, if you really want to save the amp, is to let a good engineer deal with it for you