i've always had the pmcs set up with the rockwool behind them. they've recently replaced / supplemented my BM5s, which were similarly positioned (though toed in). the bm5s sounded fine that way - not too much bass build up (though the difference in bass volume and extension - not to mention overall improvements in depth, imaging detail - between using the BM5s with my old Servo 260 and the new Flying Mole DAD HT-100 Pros is staggering, and reveals that the BM5s are actually mighty fine monitors - i'd been pretty disappointed by their performance with the servo 260!).
given this, i would hesistantly guess that the pmcs should be too badly affected. the pmcs do have noticably more bass than the bm5s, both in extension and volume. this is hardly suprising given the larger cabinet and transmission line. given that they're fairly close to the rear wall (not the side walls), i imagine that the rockwool improves things fairly significantly, though it's difficult to say i've not tested them either without the rockwool in place or with the speakers further into the room. of course, the other thing to bear in mind is that the pmcs have a rear firing transmission line, unlike the bm5s with their front reflex port, and so i expect the pmcs are even more sensitive to rear wall proximity.
if i get the chance, i'll experienment when i'm rearranging things (am waiting for some ready bags to arrive) and report back.