Update: So far so good... which is not that good a news for me (I would rather go back to a single VP-150 on top of entertainment unit). My strategy is to use this configuration for another 3 weeks and then switch back to see if I notice a difference or maybe even an improvement!
It doesn't look good for reverting though. Some reasons are hard to validate as it is pratically impossible to do comparisons (1 hour per switch is too long!) and one is unmistakable.
1) The hard to validate:
a) Overall "sounds sharper" in the sense of perceived improvement in positionning on the sound stage.
b) Upper bass is much better. That notably allows my brain to better tie the deep bass with the "location" upper bass. For many SACDs I have that I did not like as much because of "bass that seems to come from nowhere" now clearly "comes from the front" even if the bass is really all around.
c) I frequently could tell my sub was behind the couch (sound + vibration). Now, even with depp pipe organ, I feel the envelloping effect of the sub-woofer, but both vibration and deep notes now seems to be all coming from the the organ is "physically".
d) Stereo and 4.0 playback is MUCH, MUCH better.
e) There was many times (re-listening to stuff I had listenend to very, very recently) where a cello would make my gut vibrate instead of just my butt.
2) The negative: Getting up and moving about results in dramatic change of tone. Sitting anywhere on the couch is very good, but getting up to grab a beer is annoying... considering a mini-fridge to avoid getting up!
3) The unmistakable: Because of subsonic problems with unfiltered SACDs, I use two sets of programs in the D2v (Movie, Music), which is renamed in my "ultimate remote control" "Sub: Normal" "Sub: Front" where the second one is a config without sub, but with ARC+A1400 doing a great job in increasing the usable M80 bass close to 22-25hz. Anyway. With the previous setup, whenever I switched to "front", the entire sound structure would change. It seemed as if everything had moved forward and sounded thin. Very disappointing, as you can imagine as I had spent a few hours of coding to implement the feature that fixed the rumbling, but made the music sound bad. BUT NOW, whenever I switch between the two, there is ZERO difference in anything but the deepest bass. So some annoying rumbling in many recordings are now diminished (but not eliminated) without me losing anything. So far, this by itself is a winner for the M22 upside down.
This may change in the future, but as it stands now, I might be a convert to always use vertical speakers for center even when it means ugly setups. Then again, I still need to compare back with dual VP-150s because that was very, very good as seemless soundstage and I never tried by "no sub" config while I was using 2 VP-150s.