Two different types of hmmm and they are being confused here. Hmmm from the cable or ground loop or bad plug grounding is evidenced by it being audible thru the speakers and this hmmm is being introduced on the ground floor of the sound being processed.

Hmmm like Spencer is describing is known as transformer hum, it is caused by the 60Hz signal vibrating the windings of the transformer, hence you hear the hum from inside the chassis. Toroid transformers are known to hum from bad AC, hence the recommendation I made to check the power.

Outlaw appears to use pretty high quality toroidal transformers from the pics i have seen of them on the web, but then again, one could be defective or your power could just be really nasty.