I've never heard M-Ss - how do they compare to your Monitor 7s?
The M7s have an overall bigger and fuller sound, but the C6s are actually very nice-sounding speakers, with good imaging and a large soundstage. The highs are very similar to the M7s, which is partly why I like them. Mids are reasonably good. Bass is a little light, but I clicked the bass control up a couple of notches and the low end really came to life. All in all, very good for such small towers and for such a relatively small price.
Also, why weren't Studio 20s (or 60's) on your shopping list?
I considered both, but I didn't want to get into stands, which ruled out the 20s, and size limitations ruled out the 60s.