Exactly...so I buy 2 pair of the 9.1, a center and a sub........
any issues?
How about experiment with 3 pairs of 9.1 for 6.1 channel configuration? I haven't tried it but if I have to build my HT system all over again, I would choose 3 pairs of bookshelves and a very mean sub. I don't know how much 9cs costs but if the price is almost the same as 9.1, I would buy 9.1 instead of 9cs... that way I get more value for my money...2 speakers instead of 1. hehe
By the way, having 5 (or 6) identical bookshelves are ideal especially if you're into multichannel music. Reason? Seamless integration. You are assured that your center, left and right speakers should sound the same (syempre same speakers e) plus you get a center back as a bonus. Take note also that you'll only find movies encoded in 7.1 channels in BluRay only. For DVDs, maximum is 6.1 discrete channels. If it will sound good in multichannel music, there's no reason why it won't in HT as long as your system is properly calibrated as suggested by blued888.
Another justification for getting all identical 5 (or 6) speakers would be to mimic the speakers used in recording companies. Sound engineers mix surround music using identical bookshelves (not floor standers mind you)...that way you're assured to hear the effects as the sound engineers intended when they mixed them.
Spend more for the subs. Velodyne is a good brand. Get one that would literally shake your house. hehehe