center spk for JBL s8r

I have a pair of JBL S8R 2 channel system and it has been reproducing great Stereo sound from my Linn LP12. I am recently thinking to add a pair of rear and the center speaker for the new tech SACD and vedio A. I am looking for the help if there is anybody had experience on this. What could be the chosen center speaker and rear bookshelf speakers (I do not have much space in the rear for another pair of S8R or 4410/4408) to match this huge front S8R speaker pair. I will appreciate.
I built a small box center-channel speaker that sounds very similar to your JBL S8Rs. I use a 6-1/2 inch Fostex woofer, a JBL 175 driver/horn/lens midrange, and a JBL ring-radiator tweeter. Use car speakers for the rear-channels to get good bass in tiny enclosures. The perfect surround-sound speakers to match the timbre of the JBL system are 5-1/4 inch Helix coax speakers. They are German-made equivalents to the old Phase-Linear speakers, and sound much better than JBL car speakers.
Speaking from the position of someone who wants to see the best produced from any system, especially a MC system, and has encountered this scenario dozens of times, IMO if space is an issue for the rears, then maybe rears aren't the best option for you at this point. Besides, your be supprised of the result by adding a center speaker as opposed to only 2.

How much space are you refering to..ball park figure?

Depending on desired location, it may not be practical. However, its possible to achieve adding rears with not much space at all. It all depends on how much space is available.

A compromise of car audio speakers as rear is not the best way to go. If your not gonna use them as the front, then why use them for your rears?

As for the center channel, its always best to use a speaker as close to the fronts as possibe. Another S8R would be ideal.

If you want to bring out the best in your MC system, then the same precision used to build your 2CH system should be use to build your MC system.

Shoot me an email whenever you have some time.