hmmm, maybe i'll offer another different way to look at this. How far apart would the M22s be? If there is a large distance between them, maybe it would be prudent to purchase the center channel first as it would help create a more realistic front soundstage. On the flip side, if they will be located close together, you can use a "phantom center channel" setting on your receiver and i'm sure the M22s will fill in admirably. (IMO, i'd probably go for the sub first as that will allow more enjoyment of music too).
good luck
Scott
