Thank you everyone for your replies:

Gary - what you say sounds like the best idea, as I did try using a two prong just to test, and it eliminated 80% of the hum. If I only knew what a volt meter is. I'll have to ask my friends - hopefuly somone has one.

Graham - I've isolated the offending machine - its my dbx compressor. But I don't understand what you mean by "disconnecting" the equipment. If its disconnected, it can't be used - which is not exactly a solution. Additionally, every machine needs its own grounding as they are only connected via audio cables - which do not pass on the ground.

Fred - thanks for the idea. If I'm desperate I'll do this. I'd rather not have extention cords running through my hallway!

BN - Event Precision 8s. Originally I was thinking of spending $300 on monitors, but I wasn't satisfied until I heard these. I closed my eyes, swallowed, and laid the money down, and try not to remember that I ever had it. I have learned so much about sound just by listening to these beauties. http://www.zzounds.com/item--EVEASP8

Anyone with more ideas - please let me know!