Are you saying that you totally disassembled your components and then added them back into the system, one at a time, and you didn't notice the reappearing hum at the time the offending was reinserted into the system? I agree with the above re ground loops v balanced.
If you start with sources, removing one at a time, you should notice when the cause disappears. If not, then when they are reassembled, one at a time starting at the amps it should appear. Just let some time lapse between each change - sometimes hums take a bit of time to reappear due to warm up issues, etc. (I have amps which hum immediately, and others that start to hum after substantial warm up. The former I have been able to 'fix' with cheater plugs, the latter not at all absent some overhaul, if that, but much depends on level and speaker sensitivity.
Once you ID the source, the solution is probably a walk in the park.
If you start with sources, removing one at a time, you should notice when the cause disappears. If not, then when they are reassembled, one at a time starting at the amps it should appear. Just let some time lapse between each change - sometimes hums take a bit of time to reappear due to warm up issues, etc. (I have amps which hum immediately, and others that start to hum after substantial warm up. The former I have been able to 'fix' with cheater plugs, the latter not at all absent some overhaul, if that, but much depends on level and speaker sensitivity.
Once you ID the source, the solution is probably a walk in the park.