Speaker hum

I have a NAD T765 receiver (less than one year old) that i run through Quad 12L2 speakers and MIT Exp1 cables. Sounds great on 2 channel music. When i have a high wolumes music is very clear and dynamic (no clipping or anything), but a staticky, hiss comes from the speakers during quiet periods in songs or between tracks. What would be the most common cause of this ? Assuming from the power supply/amp, but wondering of power conditioning helps this sort of thing.
If your receiver was tubed, I might say it is a bad tube. It sounds like RFI which can be hard to track. It can be caused by computers of televisions creating interference with the signal to the receiver. Make sure the cables aren't resting too close to each other.