To give some follow-up, I've experimented a bit. The hiss just barely registers on a Radio Shack sound meter if you hold it right up to the tweeter (which puts it around 50 dB.) This is with Paradigm Studio/60s, which the manufacturer claims have a sensitivity of 90 dB (room).

Compared to my old setup (using the built-in amps on my Denon 3802 receiver) there's a bit more hiss when the sound is muted or at low volume, but less when the volume is turned up (since the 200's hiss is constant while the 3802's hiss vaires with the volume setting.) I haven't been able to hear it from my listening position 9 feet away.

Adding noise filters to the power line (sub-$100 noise filters anyway) didn't seem to change anything.

I've read that all amplifiers hiss, it's just a matter of how much and whether it's a constant or varies with volume. I'm not sure this degree of hissing is a problem; it's quieter than other noises in my listening room (e.g. the whirring of my CD player, the fans in my ReplayTV and Samson S1000, etc.) If I had particularly sensitive speakers and/or sat closer to them and/or had a listening room that was dead quiet I'd be more concerned.