Hello,
I've heard various people mention the word "noise floor" and from what i gather it is the lack of "hiss" emitting from the speakers. I've checked into this on my system, putting my ear right up to the tweeter, and to my surprise, there is a noticeable hiss.

My question is, what is causing this? Is it the amp? Receiver? Any input would be much appreciated.

Most likely it is the receiver.

In my opinion, if it is a very high frequency hissing noise heard only a few inches from the tweeters it is either the pre-amp (receiver) or the amp. I have heard a similar faint hiss from both pre-amps and amps, using different components and swapping them out. It never made any difference when I was listening to music, however.

There are many reasons this can show up. But with proper grounding and noise protection, you can probably narrow it down to the receiver or amp. I had a hiss that was cause by a ground loop.

Ok, I unhooked the interconnect from the amp and put my ear up to the tweeter and still there is a slight hiss. I'm guessing then, it is from the amp. This baffles me though from reading Gonk's review of the 990 he mentions no hiss at all and he's using the 750 amp. I would expect the noise floor to be the same between the 7100 and 750 amp? Hopefully Gonk will chime in here....

Hiss can be from a component, or it can be from wiring. Just a thought, but do you have any power cords near your speaker cables? If so, you might try segregating them a bit.

No, the power chords are at least a foot away from the speaker cables.

By the way, my rears are completely silent. There is only a slight hiss from my fronts and center. Probably not worth worrying about but got me curious when I saw you mention no hiss from your 990/750 combo. I would assume the 7100 produces just as low noise floor as the 750, but perhaps not?

Look for something above or below the speaker wires that is common to the front. Even a refrigerator the floor above, if the wires run through the ceiling can add noise. Since the back channels don't get it, it may be route related..