I own a pair of IB2's (not IB2i's or SE's) and I've heard the ATC SCM50A's many times. I've also heard the passive ATC SCM50's a couple of times.
The Advanced Transmission Line IB2's are of studio heritage and were designed for accuracy, neutrality and high power handling. They're not tweaked to sound 'nice' (you can watch an interview with their designer Pete Thomas on YouTube) and are therefore very demanding of what they're fed. They'll reveal as much what may be wrong with your source/s and amplification and overall set-up as what's right with them. If you can get all those things right which, as I've found, is by no means a doddle (it took me three years), they can be very rewarding.
Opinions seem to be mixed as to whether the IB2i's are actually better than the original IB2's (though Pete Thomas would probably declare unequivocally that they are). I couldn't afford a new pair of either, but I was extremely fortunate to get my IB2's pre-owned in mint condition at a steal of a price. (That done, the next thing I had to do was replace my Bryston 4B-SST with a pair of 7B-SST's to give them the muscle they demand. The 4B's just couldn't hack the levels at which I occasionally like to listen to certain material).
The IB2's also need quite a lot of elbow room if you're to avoid problems with their deep and potent bass. For me, the only solution was a pair of subwoofers fed via the excellent Bryston 10B-SUB crossover.
The ATC50's (10" bass drivers) and 100A's (12" bass drivers) are ported designs, incorporating ATC's own amplifiers, which, for both models, provide 200w to the bass drivers, 100w to the midrage and 50w to the treble. As far as I can tell, rather than three separate amplifiers each speaker contains one amplifier with three separate output stages but, to my ears, the actives have always sounded hard, steely and transistory.
The passives are much smoother and much loved by many users, but the 50's don't go as deep in the bass as the IB2's. The ATC SCM100's are bigger and may well match the IB2's for bass reach.
The PMC's can be driven actively with Bryston's Power Pac modules, though I don't know how many you'd need to match a pair of outboard 7B-SST amplifiers (900w each into 4 Ohms) or even if you can fit more than one Power Pac module on the back of each speaker.
I know very little about PMC's MB2's, except that they're even more expensive (and bigger and heavier still), though I imagine they were designed to be even better than the IB2's.
One other thing ~ PMC's IB2's and ATC's (passive) SCM50's weigh 91 lbs each (which is about as much as I can move by myself). The SCM50A's and both the SCM100's are heavier still.
If you can live with the amplification inside ATC's active models, compared with their PMC counterparts with (suitably muscular) outboard amplification they're actually relatively good value.
At this level of outlay, I'd recommend careful and thorough audition of all options, even though you're unlikely to find any dealer that stock both PMC and ATC.