I've owned both PMC IB1's and IB2's. I sold my IB1's and kept the IB2's. For context, my room is designed by Rives, so it is heavily treated acoustically speaking, to get the most out of my gear (see gallery for some photos), IB2's are tri amped (Bryston 7BSST's on bass driver, 4BSST split between tweeter and mid range) and speaker cable is the same cable as that found within the speaker cabinet itself - works better than any of the higer end cable that I've tried.
As you know, the primary difference between the 2 speakers is the mid range. The secondary differences are the crossover and cable terminals at the rear of the cabinet.
In terms of listening, the biggest difference I found was added resolution in the mid range which managed to bring the performance more into the room. It seemed less about listening to a CD and more about listening to live performance. I realize that there is quite a bit of cost difference between the 2 speakers and it's not going to be worth it for everyone but if you value this kind of added intimacy with music and you have a high resolution system/room, I highly recommend the IB2's.
Best of luck with your decision.