There is a 2 block area downtown here where everyone gets a radio station on their amp. It doesn't matter what brand of amp you have. Dave's right, the cord is acting as an antenna.
Radio Shack's RF choke cut it down to a manageable level for me. It's just a square of ferrite material that you wrap your patch cord around and locate it as close to the amp as you can. MCM, or maybe Mouser, I forget, has some cute little chokes that clip on your cord, much neater.
I recently had the 60hz hum problem too and found it was the cord. It was brand new (not cheap either) and measured fine with an ohmmeter, but, you know....
Anyway, good luck.