Try elevating the cab -- on a stand, if you have it, or something else that'll get it a foot or two off the floor. Mike would usually play the cabs this way, I think -- at least he told me that was his preference with many of them (including the 46, though my experience is with an earlier model than yours). If you can't elevate it, try tilting it up on a wedge or even setting it on its side (though the dispersion won't be quite as good that way).
What I suspect you're hearing is an excess of coupling with/reinforcement from the floor, which would mask your mids and highs. I felt this way with my 46 sometimes (and Mike always advised me to elevate it). On the plus side, the MAS cabs don't get neutered in the low end the way most do when elevated. I think that you'll find you still have plenty, with much better clarity, if you try that. (Note: I am assuming that you have the rectangular, "2x2" profile 46, not the column version, which I've not played but would imagine is to be set on the floor -- other cabs, like my MAS 112, do fine on the floor, too.)
The other possibility that comes to mind is that maybe you have too much cab for the gig/room -- I don't know this, of course, but, if you're playing at very low volumes with the MAS cabs, they don't really open up and do their thing properly -- the rear wave/vortex contribution isn't in its sweet spot (which, to be fair, is pretty broad) and, again, the lows will mask the mids (at least that's been my experience).
To put this into perspective, the effect isn't that much more than with a lot of standard cabs that need just a bit of volume to "wake up" and sound and feel right -- and the tradeoff seems to be that the MAS cabs stay more dynamic (less compressed) and open toward the top of their volume range than most cabs do, but it can be a thing -- that's why I no longer practice through my MAS 112 in my apartment: it's nowhere near itself at apartment-practice levels. Even my 46, when I had it, wasn't quite there (again, apartment practice levels). This is something I discussed with Mike and he told me that the more recent cabs were more small-space/low volume friendly than mine -- and those I tried at his place gave me that impression -- but I've only gigged the ones I've owned.
The 210 Flex Mod (run with and without its sub), MAS 46 (run with and without the sub from the Flex Mod), and MAS 112 I've had (still have the 112) were all more clear in loud, busy mixes than any other cabs I've played, but I did have a frustrating experience with the 112 (which would be like a 66 in terms of output), once, bringing it to a too-small gig, and I could see situations where a 26 or 18 might work better than a 46.
I've never used an AI head with the MAS cabs, but they always seemed to pair well with Mike's EA Doubler.
Don't know if any of that helps -- hope something in there does!
