I'm not exactly sure how JL decides on how much air space they should use. I know that I use a method to arrive at a Qtc value. The lower the Qtc (to a certain point) the better. I've tried out the same sub in varying Qtc systems. At .8 Qtc, it sounded like butt, the low end dropped off severely and the sound was boomy and not tight. I put the same sub in a .65 Qtc system, and it sounded sweet and low and tight. I'm sure if the Qtc got too low, it wouldn't sound right and not handle much power. Basically, the larger the box, the lower the Qtc. The Qtc values that a 2.5 box on the JLs should be around .75 or .8
I'm sure is sounds good, but wouldn't it sound better in a lower Qtc system? I'm sure every subwoofer has its own characteristics, but I"m just tossing this info as something to think about when deciding on how big of an enclosure you want. Every time I've built a sub, I went just a tad larger than manufacturers specs and been very happy every time.
DEQ-P9 (30band eq L/R, time alignment, 36dB/oct 4 way crossover)
Soundstream Tarantula (for subs)
Precision Power PCX-4125 (for horns and midbasses)
Image Dynamics CD2 comp mini horns
Scan-Speak 7" 18W/5535A midbasses
Subs still pending.