I have a dual 12" (1.25 cubic foot per sub) sealed subwoofer box. I happened upon two 12" Infinity Kappa 120.3DVC subwoofers for really cheap. So I installed them, running them off a JBL gto75.4 amp. I noticed Infinity suggested 1 cubic foot per sub when it came to a sealed box. While my subs hit hard they are not as smooth sounding and do not shake everything like a Rockford Fosgate stage 3, Audiobahn, or mtx subwoofer. Could the extra .25 cubic feet of space be affecting their sound; am I not running enough amps through them (subs have 300watts rms, I am pushing 278 through each of them), or do infinity's just not perform that well?
You won't notice an SPL difference w/ the smaller box, but you will get tighter SQ out of them. An additional 22 watts RMS per sub isn't going to really make a difference either, maybe a fraction of a decibel at best. If you want SPL subs, you'll need to look elsewhere, the Infinitys are nice for SQ, but for SPL they don't impress too many people. More for the casual user not interested in hitting that hard. It's also dependant on what kind of car you have, some simply aren't as loud as others due to gain, you may notice totally different results in something like a small hatchback.