I was thinking self powered I actually own 2 old Yorkville 18's but not sure if they are up to task..yes I have a DBX crossover and a qsc 2450 ( I think have to look)
Have you tried the 18's you've got? I'd try that first. This is what I'd do if I were in your situation:
If the subs are 8 ohms impedance then I'd bridge the QSC and run the subs in series to get the most power to them. I'd also keep the subs together, either both stacked or side by side on one side of the stage, preferably center stacked. The 50hz low frequency filter would be engaged.
If there is a spare auxiliary send on the mixer I'd use that as the sub send. I'd run the main send mono, to one side of the crossover, then to the amp powering the Carvin 2x15" tops. Then the aux side would go to the other side of the crossover and to the QSC powering the subs. Crossover set around 90hz, and only the instruments with legitimate content below this frequency would be sent to the subs: bass, kick, floor tom (maybe), keyboards (maybe).
There's a lot missing from that scenario, but hopefully that gives you an idea how you can most effectively use what you've got.
--------------------
Regarding a new, self-powered sub for ~$1000...I don't think that there is much out there in that price range that will be better than optimizing your current gear. A single high-output power sub suitable for loud rock would likely run several (or many) thousands of dollars. The Yorkville sub mentioned typically runs a couple hundred more than your indicated budget, and while being a fantastic performing sub for the price I doubt would keep up with a loud rock band for anything other than a small crowd and venue.
