Sunfire Sub Hum

Isolation transformers sometimes ( not always ) degrade the sound.

Sounds like a ground loop to me. I solve this problem using a Jensen Transformer (about $200) that I bought off of their website (don't remember site address). On the cheap you can buy an isolation transformer from Radio Shack. It is sold as an auto product but works at home as well (about $20). May want to try that one first as it is easily available and can be easily returned if it doesn't work.

Oh I forgot, if you have your TV in the same rig, your antenna may cause a ground loop. There are gadgets on the market to solve this problem.

If I had your problem, I would at first do as you have done. If of no avail: 1. check if your sub is plugged in together with your other gear on the same line. If not, you may have a ground loop. See that all gear is fed from the same line. 2. check if your interconnects to the cube are properly shielded annd if the solder points in the plugs are still OK. 3. check of you have powercables close to or running parallel with your interconnects. 4. Try grounding your preamp. 4. Try to run an extra ground from one of the woofer's screws on its backplate - not from the any of the inputs though. More does not come to mind at the moment. Good luck!