OK, I've done some more testing and reading up since this thread and here are all the current details I have, not sure how relevant they are, so I'll throw in everything. It is not necessary to read that above thread. Please help me out, I'm very desperate to fix this.

My Roland Cube 30X is making a humming noise when all guitars (Gibson SG standard 2009, and Yamaha RGX520FZ). Both guitars have humbuckers. The humming is significantly lower on the yamaha, to the point where if I didn't listen for it I doubt I would have noticed it. When touching any metal part, the humming will significantly reduce. The only cause of this I found that sounded reasonable was that something was wrong with the circuit, and something was not properly ground. Here are some situations:

For both guitars, the hum is worst for the "middle" volume of the guitar. When it's really high or low, the humming is reduced. When the volume is off, the humming stops.
The humming is directly proportional to the amplifier's volume.
It doesn't matter which pick-up is being used, the humming persists.

I assume it's something wrong with the guitar, but both guitars have it and I've tested (noobishly though) a lot of my SG with a multi-meter to see if anything wasn't connected and everything seemed to be (including the bridge to the pots). I have not had the opportunity to switch out the amp or cable yet though. The house is only 7 years old, so I doubt it's the electric wiring here. I've also tested it with the multi-meter and am reading 120 volts from the outlets, which means the outlet voltage is proper (and it is ground properly).

If you have any questions, please ask. I would really love to get this fixed, and I'd rather not have to go to a professional, but probably will if I can't fix it myself.

Thanks a bunch!!
I actually am by flourescent lights. If they're off, will that still be a problem?