Where are you planning on positioning your sub in relation to your JBL's? Are you planning to put it in the corner, or are you going with multiple subs placed under or beside the 4655's? If you must go with Eminence drivers, here's what I'd recommend:

Eminence Kappa 15LFA (Adam is very familiar with these).
5.33 cubic feet tuned to 33 Hz will give a nice gradual roll-off with nice extension and relatively deep bass. Remember, this is a high-efficiency driver, so you can't expect sub-20Hz bass out of it.

Eminence Delta 12LFA
A 3.2 cubic foot box tuned to 38 Hz will give a pretty decent -3dB point of around 37 Hz. With room gain, you could probably expect good bass down into the low 20's range. Get two or more of these woofers and you'll have some excellent SPL.

I seem to remember reading Wayne's comments somewhere saying that Pi-Align is NOT really intended for subwoofer design. I feel the same way. Because a true subwoofer is really only intended to cover the bottom octave or two, the rules for successful design are quite different than for a full-range system. Generally speaking, deep bass SPL is easier to obtain from lower efficiency, low-resonance, high-excursion drivers used in multiples and/or with LOTS of power. The LF and LFA drivers in each Eminence range are a good compromise between high-efficiency and low resonance. I think the Delta's and Kappa's are the most user-friendly.

Good Luck!
Colin