Put a JBL 2245 18" in a 8' box
put a JBL 2226 15" in a 3' boxthis will get you from 25hz to 1600hz with no problems.
vented or sealed, you weigh the pros and cons and decide yourself.
if you want clean fast bass, those two drivers are among the best.
If you run active amplification you'll have a great system with the other drivers you have now.
Bill