M2s appear to be a good fit, but having been in your position, I'd just say be sure to think long-term. What do you want to own in 1, 2, and 5 years? If you surf around here (and re-read some of spiffnme's posts about purchasing advice) you'll see a common theme: Beware of underbuying, which can lead to speaker-envy and costly readjustments down the road. To an extent that's the situation you're in now (a place all of us have been, so no shame in that!!!!!): You've underinvested in some equipment you want to move up from.
With this is mind, consider carefully the immediate and future uses of the new mains you want. If you think they might one day end up in the bedroom or an auxiliary room as stand-alone music mains, then the M3s might be the best choice. M2s could also serve that purpose, but the consensus of reviewers and forum members is that M3s are often a better standalone music speaker in the small, sub-$300 range because they create a fuller sound. If you want to gradually move up to an upgraded HT (new sub, center, and MAYBE surrounds, though your use of old speakers is to be commended here), then you may want to hold off until you can spend $400 and go with the M22s.
If you're seriously looking at the M2s, then I'd suggest you save some sheckles until you can get the M22s, unless you foresee now an auxiliary future use for the M2s. Otherwise you may just end up with too many small speakers. Given your budget and aims (which I can relate to), you might find that the extra $145 for the M22s save you money in the long run, since they'll prevent you from wanting to upgrade your mains for an indefinite time period. (Heretical I know, but one other well-priced set of mains to consider are the Ascend Acoustics CBM-170 -- lke Axiom only available online, highly accurate, aggressively priced, and well loved by hifi reviewers.)
Final thought -- M22s, or M3s, will probably give you more pleasing music listening, given that your Polk sub may not be something you want to use too much for music. So to the extent that music will be on the system, the M22s and M3s are again worth considering over the M2s. If you were going to build a complete HT with 5 identical speakers (or 4 identical and a new center), then all M2s with a VP-100 OR a 5th M2 as a center would be a great option.
The M2s, incidentally, appear to be outstanding speakers, so I'm not knocking them, and I've never had a chance to hear them either. I just know that in my similar situation, I would not buy them as mains unless I foresaw another use for them, which in my case would be as future surrounds coupled with some M22s or M60s (which I presently own and love). If I foresaw coupling M2s with M22s as surrounds/mains, then why not save up and get the better speaker first?
My two cents.
Birdman
