Normally if a cap is shot , you'll get a loud hum. It's possible your bridge has a shorted diode and is leaking AC thru to the cap which will still filter a bit but presents a short to the AC output of the power transformer. The way I check is to look for AC leakage is to use a scope , but you can probably get a reading on a VOM . Put it on the AC scale and see if you get any reading. Caveat is even a low level of AC past the supply should produce some hum, and will produce effects on both channels. It sounds more like adriver or output stage prob. Is the amp discrete or IC outputs?