Subwoofer cable picking up noise

[Chrispy]

Whether it is effective depends on where the noise is coming from. If you still have loud hum with only the preamp and sub connected and both plugged into the same socket, then I'm not socket, then I'm not sure what you can do.

I'd still think lifting grounds to isolate might be an idea. Not suggesting long term use, but it can work if you're careful