My suggestion would be to go work from the speaker backwards. Disconnect everything but your speaker and power amp, short your power amp inputs. If your hum is still there, it's the power amp. Try lifting ground in that case. Work your way back through pre-amp and source.
A note of caution here; there is a reason why this stuff is grounded, it is for your safety, so be careful.
A note of caution here; there is a reason why this stuff is grounded, it is for your safety, so be careful.