So I have an old Pro Power Amp. that was on a shelf for a couple years due to buzz every time I tried to hook it up.
Did some googling for possible fixes "finally". Ran in to a Video of some one simply opening an Amp. and dusting it out with a compressor presto hum gone.

So I decided why not. Don't have my own compressor but my shop vacs blowing side worked great. Started by sucking all the crap I could first then the blower. Highly recommend for any one whom finds a hum somewhere down the line in your system. A really fast easy first step.