I have an MT2400 that induces a hum not so much in the speaker - but moreso back in the source line level signal. I need to troubleshoot to rule out a ground loop.... but is there anything else to look for? Any caps or power supply issues to investigate in the input stage that tend to become problematic as it ages? Outputs should all be fine... I believe mfg is early 90s. Cosmetically, the internals look clean and caps look OK.