You guys are underestimating the JBL drivers and giving waaay to much credit to the XS30. First of all, being a sealed unit, the XS30 isn't going to have that much more deep bass than the JBL 4645. Second of all, whatever deep bass it does go for will be drenched in distortion. Third, like I keep saying, there is more to performance than sheer output. JBL makes some really good drivers and the 2242 is expensive and sought after for a reason. With a 24 BL motor and 167 gram Mms, the JBL is going to be tight, a lot tighter than the PSA drivers will be. It will likely have superior transient response, better cone control, and tighter sound quality. It also has a nicely linear response all the way up to 300 Hz, which is a definite advantage for a behind the screen subwoofer. Even at tremendous output levels, the 4645 keeps the distortion lower than 10% at 50 Hz and above, which is definitely not something you will get from the PSA sub. This also means the OP could cross it over higher and relieve his mains of playing back deeper bass, thus reducing their distortion as well. The only problem is it just doesn't do deep bass.
Big deep bass will be hard to achieve on a $2k budget, especially a sub that can keep up with 3722 mains. I would look at building some gjallerhorns or some othorns or a Tuba HT sub. Look around the DIY section, ask some questions. Very few commercial subs would be able to keep up with those JBL mains with respect to deep bass. Some JTR Captivator 2400s might or some Funk 18.3s.