Along the lines of Paul C's reply, I was going to mention the "Open Loop" noise of a typical Amplifier.
By this I am referring to the signal input loop being open (no connection to equipment), and not "driven to ground" as is done through a volume pot on the equipment being amplified.
Best example is a Guitar Amp(and most specific - a Non-Humbucking pickup!).
Without touching the strings, the Amp will be picking up all the stray 60Hz noise that's present.
Pull the patch cord's male 1/4" phone end from the Guitar, then place a finger across Tip and Ring; funky noise will be Amplified.
The volume pot on the Guitar will drive the power amplifier's preamp input stage to ground as the volume level is decreased (via it's internal resistance). At the end of the taper (turns), the preamp is driven to ground through a resistance of something like 250KOhms.
If the Amplifier in question DOES have its signal input connected properly and is driven to ground level, then verify the input line (signal level) is solid and hot!
Look into power quality issues only after verifying the signal input(s) are proper - as that's where the noise is coming from!
Scott35
