I did a pretty thorough in-home demo with the M3, the MA Bronze 2, and some JBL E30s.
The JBLs were pretty bad in comparison to the other two. The Monitor Audios went deeper than the Axioms (the S2s will go even deeper judging by my in-store comparison of the B2 and S2) and they had a little softer more forgiving tweeter. But the B2s also have a little boxiness and kind of a nasal, resonant midrange.
The M3s, of course, were detailed, open and transparent, but lacking in bass in comparison. Mids were much better. Highs could be a little fatiguing on poorly mastered/recorded material.
In the end I kept the MA Bronze 2s for a number of reasons: they are in a "hard" secondary listening room (just a drywalled square), no sub being used, I ordered the M3s in the wrong finish for my decor, and because I have a roomful of Axiom speakers downstairs already. I'm pretty sure I could have been just as happy with the M3s (and saved a few bucks, too), but in your case I don't think the M3s vs the Monitor Audio S2s is a fair comparison. The price difference is considerable, eh?
And I'll second bigjohn's opinion of Boston Acoustics - ick!
