I guess I have something against JBL due to my experience with some of their pro stuff. I've worked with their top-end portable systems (passive speakers and powered subs) and neither impresses me much. The subs don't go very low at all, and just tend to sound boomy and undefined. And these are 18" subs. Call me a cynic, but I don't think that 35Hz is very respectable for a massive sub.
Of course, I do realize that they are designed for live settings where music doesn't really go below that, I would think they should be capable of lower frequencies. But perhaps if they are tuned higher they can achieve maximum output at desired and useful frequencies. I honestly don't know (WmAx?).
I just think that even when going with a custom install, you can do a lot better than a synthesis system for the money, meaning better bass depth, better sounding highs (while I haven't heard one personally, I do know what JBL horns sound like, and it's not to my liking), and so on.
And hell, if you wanna talk about a manly dual 18" sub, let's discuss the Wilson XS. 16Hz at 112dB Anyone?