I just received my M2Ti's and have them playing with a small (and cheap) Pro linear sub. The Axioms sound great, but I've got two issues with the sub:
1. The crossover (at its lowest setting of 50hz) is letting very audible sound through >100hz which is seriously overlapping with the Axioms, not to mention localizing the sub.
2. The extended lower frequencies are not as tight as I wish they would be making everything sound just a bit boomy.

So as you can see, I'm not 100% satisfied with the setup, but I'm limited by a tight budget. I've been thinking about returning the sub and M2Ti's - For the same price, I can replace them with some M3Ti's or maybe some M22Ti's if I can find 'em in the outlet. I lose the lower bass extension, but I'm hoping the better tweeter/cross over freq., and bigger driver will give will give me the sound and bass improvement that will make me forget about the sub (for now, until I can get a decent one!)

My other option is to keep the setup and place a quality passive crossover ~70hz-ish before the sub to take out that overlap. What do you guys think? Will the better tweeter and larger drivers in the bigger Axioms give me enough improvement to justify sending this stuff back?

If you really want a good combo, get rid of your sub and buy a new one.

The M3 and M22 are different sounding speakers. i hear the m2i is close to the m22, so if you like its sound, and you want to upgrade, get the m22.

the m3's have a more laid back sound

all three of these speakers do not have great bass extension and require a sub to sound complete

save your money, and buy a better sub. purchasing an external crossover is money for nothing, when you can put that money and the money from returning the sub to buying a better sub.