computer audio ground loop hum--any suggestions?

I recently installed a new power supply in my desktop. Everything works, but I have acquired what sounds like a ground loop hum in the speaker with the on/off switch and volume control (and the hum gets louder as I turn up the volume). There is no hum in the other "satellite" speaker. The power lead from the woofer unit only has a two-pin plug on it, so there's no chance of using a "cheater" plug to life the ground, although it seems as if that wouldn't have worked anyway.

Any suggestions?

(FWIW, the wiring scheme is pretty standard. A lead with a stereo minijack takes the two-channel audio from the back of the CPU to the back of the speaker with the on/off switch and volume control. Another thicker lead from the back of the speaker goes down to the "woofer" with a multi-pin connector going in to the woofer. A second thinner lead coming out of the same multi-pin takes audio out to the other "satellite" speaker. A separate power cord from the woofer goes to the power strip.)