Low hum from amp

From just the amp or the speakers as well?

If not the speakers, probably just transformer hum caused by dirty power or poor mounting of the part. Try another outlet.

If it's coming from the amp -- the physical amp cabinet -- and not the speakers, it may be the power  transformer humming. This occurs when there's a little DC offset on the line (not uncommon). A cure is available as the AVA HumDinger. I have three; they work great.