You really want the QsC KW-181 at 1204 delivered in a soft case from ProAudioStar. The QSC integration is as good as it gets... Barring that, get an older passive 18. pAS, RCF, EAW, even Peavey and selected JBL models - add an active crossover. Set it for 100 hz and goose the low's tab. That will be close to what the QSC setup is doing. And you need a decent power amp to drive it. Skip the class D stuff and go led sled.
You're going to be into this in the $700 to $800 range when you're done... Used K-Subs are in the 800 range and used KW-181's are more expensive than new with ProAudioStar. My advice is spend up to the 181 and recover. They Rock, really, really hard...