Intrepid,
Ray gives some great advice here. I've just added and edited some comments.
In reply to:
At the end of the day, the choice of M2/VP100/VP150 for a center falls to preference, budget and space. The VPs are designed to be a bit more flexible for placement horizontally and the VP150 tops the "get it IF YOU WANT THE BIGGEST EDIT DELETE WORD "best" available" rankings.
Surrounds get you the M2/M3/QS4/QS8 choice. Again, preference, budget and space. Extra considerations here 1) the Qs seem to diffuse the sound a bit better and 2) the Qs can be wall hung very easily while the Ms might require stands or the Full Metal Bracket from Axiom (= expense & lost space). Apparently, the QS4s now use the same tweeter as the 8s and this has made a difference. (only in the minds of the buyers in my opinion)
In your current room, the VP100 and QS4s make as much sense as anything. If you plan to move to a larger space later, taking the EDIT DELETE WORD "upgrade", ADD IN "larger version" of each model might be good planning.
To address you questions directly:
In reply to:
1. With either of the two center speakers, there may be somewhat attenuated off-axis response, as well as very small variations in SPL levels (not that I would even have a discerning enough ear to detect either).
The off axis response is a non-issue since as someone had stated before (perhaps Bigwill), if someone sits that far off axis, they may as well go home and watch the movie there. Off axis measurements drop for all speakers, horizontal or vertical. Width makes a difference here.
In reply to:
2. The VP100 is nothing to regret purchasing, although if more money can be spent the VP150 brings improved sound to the mix.
Improved sound? This would be more room size dependant so a better quality sound than the VP100, unlikely.
A wider soundstage and more volume for larger rooms? Yes.
In reply to:
3. Lastly, JohnK in particular (not to single anyone out, but he seems to be the most visible supporter) praises the merits of an M2 for example, in handling center duty for M22 mains.
Refer to Ray's comments above. A single M2 is small enough and cheaper than the centre channels to make this option worthwhile considering for smaller rooms or those looking at a smaller setup. I probably would not choose to use the M2 with a tower setup but since you have the M22s and a smallish room, it is an idea to think about.
