Welcome to the slippery slope. Sounds like the M2s were the right choice over M3s for you, as they were for me.
The worst description I have heard of the M2i's is "painfully accurate", ie if your recordings suck you will know about it. I find them to be VERY neutral -- it's just that many speakers are not so neutral so going to the M2s (or any similar speaker) is a bit of a shock.
I had been debating about whether or not to move up to bigger speakers in my room and decided last night to stay with the M2s. My reasoning was that with the new cabling* there is a big gap between the volume levels I normally play at and the level where the speakers start to sound strained.
If I felt that I was going to be on the edge of pushing the speakers too hard I would move up to something larger, M22s at minimum. Sounds like you also have enough headroom between normal listening volumes and the limits of the speakers.
If you think the M2i's sound good now, wait until you add a sub. It's hard to describe the difference -- it's much more than just getting some more low notes, the whole soundscape seems to solidify a bit and the last traces of "boxiness" go away.
Enjoy !!
* Just to be clear, by "new cabling" I mean cabling the pre-out/main-in through the crossover in the sub so that the M2i's are actively rolled off at 80hz -- not some exotic new wonder cable that gives me 6db more sound and tighter bass
