The OB1's would be a better solution for you. Although the CB6 also uses the same mid and tweeter as the IB1, the CB6 isn't designed to be mounted vertically so the tweeter will be offset. I've used OB1's as rears before and if you set them right in terms of delay/distance and channel level, you'll be fooled into thinking you have a 7.1 system. You'll know what I mean anyway when you take into account the imaging capabilities of PMC's. Lastly, I might add that the extended bass from the OB1's would be less of a trade off from your IB's
