Well, I fear the oscilloscope is the wrong tool for the job. I fiddled around for a bit last night and couldn't show anything conclusive with the 'scope.

So I'm going to see if I can call in a favor or two and get an Audio Precision One audio analyzer on loan for an evening.

Over on HTF we have another person reporting that they can replicate my experiment, and another person claiming they can't hear any hiss at all.

For what it's worth, the speakers I've got hooked up to the 950 (PSB Stratus Silver i's) are less sensitive than the speakers I used to use with my old preamp & amp (Klipsch KG-3.2's).