With your ear up against the tweeter you're going to hear some hiss. If it's the same in both channels so much the better. It's normal. As long as it doesn't intrude at your listening position don't worry about it.
Acceptable hiss ?
Is is normal to hear absoultely NOTHING coming out of your speakers when no information is being passed through ? Or should there be a subtle hissing when your ear is close to the tweeter ? I'm getting a slight hissing, and am wondering whether or not this is acceptable. I am using Sonus grand piano fronts / center and Sonus Concertino Rear; driven by a B&k 307.