I am obviously biased but must agree with Mike on this one (although I have not heard the AN except shows). IMO, what sets the MSB (Reference in my case) apart is, in fact, how natural and uncolored the sound is across the entire frequency spectrum as opposed to the limits of tube dacs I have heard. This is a gross generalization, but in my experience they can have very nice second harmonic distortion (pretty much at frequencies of choice based on tube selection) which can be very sexy, euphonic, romantic (take your pick), but this distortion gets old to me after a while. Noise floor is their other downfall. (As a caveat, I tend to like a a transistor in the gain section of a tube source.) The best way to put my take on the MSB is it sound like good analog with virtually no noise floor.
Small signal tubes, used correctly, for the purpose of voltage amplification do not have high levels of distortion, 2nd order or otherwise. A DAC output stage is like a preamp. Go look at some preamp measurements in Stereophile and compare tube and transistor designs measurements and you will find that they are both quite low in distortion.
From what I heard also at the Munich show, the MSB room (using MSB amps as well) sounded nothing like good analog (there was plenty around for some comparison...just not in that room).