Hello patwho,
Your system is suffering from a very common problem caused by a ground loop in your system. Here is the excellent article that Alan Lofft was talking about.
http://www.axiomaudio.com/tips_eliminate_hum.html My system suffered from a buzz issue when I added my first subwoofer and it was caused by a component cable between my receiver and t.v. The cheater plug you are using will work just great but if you do not feel comfortable using it for a permanent solution, read the above link, Mr. Lofft gives excellent advice on how to find the ground loop problem in your system.
Good luck,
Dr. House