Could be a few different things. First, is your ground good? Try and find a bolt that goes into the frame to bolt it under. Also, sanding off the paint so it is on bare metal will help also. Second, what gauge wire are you using for the amp power? It should be wired directly to your battery with a fuse in-line (in addition to those on the amp itself). Next, check the settings on the amp. Most amps have little knobs on the side for gain and bass boost, etc that you need to tune for your application. Third, did you build the box correctly? Each subwoofer has an ideal volume it requires for the box. And then check the settings on your head unit. Make sure your low end isn't set really low. Also make sure your high-pass filter (if it has one) isn't set too low. This tells the subs how high of a frequency it can play.
Hope one of these helps you. Good luck. And just my .02... I'd invest in a better amp. You have pretty good subs, and I think Sony Xplod anything is total crap. The jeep I just bought has them in the sound bar... they won't be there for long.
Also, make sure your subs wire wired in parallel and not in series.