I have a NAD C320BEE integrated and I get a faint hiss from my M60 tweeters as well. Even when CD/DVD player are not on and volume is zero. It gets a little louder (very little) when the volume is maxed. I basically have to put my ear right next to the tweeter to hear it, so I'm not too worried about it.

Axiom M60ti Hsu VTF-2 Mk2 NAD C320BEE NAD C542 Systemdek IIX Cables by Unity Audio