your really not gonna get the l7's much louder. a 12" l7 id 750rms, a 15 is 1000, although the PA amp is a piece of junk, its still probably putting out 1500 watts @ best maybe 1200. so if there 12's its not gonna get much louder. if you could do 2000 rms on a set of fifteens you ll see some improvement.
also when lookin @ amp specs, IGNORE all max ratings, they are b.s. just marketing gimmicks, anything over rms on a amps ia basically useless, as it'll start clipping and eventually fry your voice coils.
i would say you best investment would be a custom built box, preferably by you. tuned to about 33 hz with 12-14 square inches of port area in 6 cubes. if your running a prefab box, you'd be surprised how much better it'll sound in the right box