I spoke with Leonard at Sonic Studios about the analog testing yesterday... I'm not a techie, and thus didn't understand everything (ok, most things) he said, but he made it clear that some kind of input should be attached, even if there is no active signal - I'll try it by attenuating the mic signal down to zero into both the 1/8" and TRS inputs. and measuring the noise at that point with various gain from the preamp at various input settings.
Based on my experience with amplified music, that seems to be no problem at all. Very quiet stuff will be another story.