Have you modeled that optimal box size for your subs? Load the T/S parameters in WinISD and see what it recommends. The manufacturer recommendations for sealed box size is almost always rubbish. A lot of subs on the market, specially cheap subs require a pretty big box to achieve a qtc of 0.707 or less. Put them into a smaller box, and the Q goes up, and among other things with tends to dampen the deep bass response. Granted, 1cu ft should be acceptable for a lot of subwoofers, but I have no idea about your kickers. Next, is of course, the issue of power. Some drivers just have a much lower sensitivity.