What is causing the hum?

I hooked up my MC-12 to my Sherbourn amps for the first time last night. I noticed a very annoying hum and also, it seemed like I was not getting all of the power I needed to my Thiel Powerpoints. With the volume all the way up, the powerpoints were not overly loud. I would expect them to be, as the Sherbourns are 200watt amps at 8 ohms. Heck, my Yamaha receiver made the Thiels extremely loud at 50% on the volume control.

What gives? How do I get rid of this hum?
Well done, Steve. I've been fighting an annoying hum for quite awhile now. No one ever explained the problem or suggested the solution more clearly than you did. Thanks. I'll give it a try.
Steve: I am printing this out to keep in my file of audio tips. Good job!
Hate to be another "me tooer", but thanks Steve - that was a great explanation.
The fact is every component that has a grounded plug should be used as such. PERIOD. The real problem lies in creating a GROUND LOOP when connecting different components into different outlets. Solution: Plug everything into one outlet if possible via a power strip or line conditioner or other device. I use my A.C. circuit which is a 30 Amp line with nothing else on it. When I want to listen to music I simply make sure the A.C. is off. This method will produce the best results. Some components rely on their ground to shunt noise, DC, whatever to. Many components will sound terrible without a proper ground. ALWAYS USE GROUND IF A THREE PRONG CORD IS SUPPLIED.
I have to add the source of most "hum" problems is in the branch wiring of the home. Either improper bonding and grounding of the branch circuits or a "load" on the electrical system that is emitting noise.