It could be the M22v4 is more linear, and hence introduces less harmonic distortion, which you interpret as less involving.
I've tried 5-channel stereo and find it kills the soundstage. PLII works very well for a lot of tunes. Even though the 160v4 is great, I find many songs benefit from PLII with no centre.
If you can, even temporarily, move the M22s into the room and see how soundstage width and depth may differ between the versions.
As for the M5, my theory is it was on the roadmap for a long time but a few things had to come together for it to be a worthy introduction. The HP driver, the v4 tweeter, improved mid-woofer, improved understanding of The Family of Curves, improved cabinet bracing and improved cross-overs all were necessary to get to the M5.
The M5 is, in my opinion, the most refined speaker Axiom makes. It takes very little to get them to disappear. The soundstage and imaging is absolutely spectacular rivaled only by my actives. Unfortunately that HP driver's coil is massive and is loaded with inductance which makes it unusable much above the crossover frequency of 250Hz. Therefore unlike the non-HP woofer, it can't be used to aid the mid-woofer. This lowers the sensitivity of the M5 requiring large amounts of power to drive it.
The M5 is truly a marvel and an absolute bargain.
