Do I have a good sound system in my truck?

The previous owner of my truck replaced the door speakers with toxik door speakers and a couple 12'' kenwood subs.. It sounds alright, but when i crank it up it the subwoofers get pretty washy. Is it just a cheap system or possibly a hardware problem. the subs are Kenwood KFC-W3003 (http://www.crutchfield.com/S-D3cDjHFfnOr/p_113W3003/Kenwood-KFC-W3003.html) and the amp is a Kenwood KAC-6202 (http://www.crutchfield.com/S-nP2JGKTxPLz/p_113KAC6202/Kenwood-KAC-6202.html) It's hooked up with 4 gauge wire.. deck is alpine cda-105... also, when I pulled out one of the subs from the box to get the model number, i noticed that there were some seams that haven't been glued properly, leaving a gap.. does that matter? does it have to be completely air tight? The box sits under the back seat.. 2005 dodge ram 1500 ext. cab.. Would a larger amp make my subs sound better?