FWIW, I've just spent hours and hours
in front of a V1 M I and a V2 M II.
At first I heard all sorts of differences.
Then I realized how directional these speakers are. By the time I mounted the two amps so they were at the same distance and angle to my head, I came to the conclusion that, other than a slight boxiness on the smaller amp, and I do mean slight, and the extra wattage + speaker on the M II, giving it a slightly sharper/stiffer sound, the two sounded remarkably the same on the same presets with the same guitar, cable, etc. And the compensation they applied for the physical differences is so amazingly effective, that even with the larger/smaller speaker, I doubt I could reliably pick out the right one on a blind test.
I think that unless people are taking all these precautions, the comparisons are mostly, if not entirely, placebo effect.
I do not hear "greater definition" in the V2 modeling, and all the tiny subtle differences I hear (and then don't) are well within the limits of position, speaker types, enclosures, mood, and other hardware issues
The guys who programmed the firmware must be laughing their butts off reading the reviews talking about "clearly more advanced modeling" on the V2. They had room on the ROM so they added the smart harmonizer, and a few stomp variants. The amps they added are minor variations on their (very good) tube model, the old ones I believe are unchanged. They fixed the Fizz, apparently, somewhere in the D/A power amp interface on the M III and up. Even if that had only required adding a capacitor, recalling and fixing all those V1s would have cost a fortune, so it's clear why a V2 was necessary.
If anything, I like the little M I a little more, because the smaller enclosure gives it a certain charming "old amp" boxiness. Adding a touch more midrange on the M II, even that seems to largely wash.
[later] Found one difference. The Pitch shifter LEVEL on the V2 takes it all the way to 100% effected signal. On the V1 even at full on, it only reaches around mixed half and half.