Hi X,
Keeping the Center speaker as close as possible to the left/right speakers is really an important issue. Maintaining similar polar response is very important.
At the recording studio level the front 3 speakers are ALWAYS the same. (and as you know I am a big believer in what the recording engineer INTENDED you to hear).
So given the choices you have go with the used IB1 or the CB6 (the mid and top will match your IB1's much better).
james