Here's a thread from the 7100 saloon that may interest you:

http://ubb.outlawaudio.com/ubb/Forum19/HTML/000022.html

I don't know if the final solution in this thread is neccesarily the only way to fix the hum. I bet the unit mentioned fixes some electrical problem that's inherent in the wiring of that house. Just first try to rule out all other possibilities before investing in a quick fix like that. Also, I've never read anything about the 755 having a hum. Good Luck!