If there aren't any local db drags going on around your area just call up the car stereo shops some that do hold the competitions have spl meters at the store and i'm sure for a couple bucks if not free they should let you see what your system can do. right now i have a semi sq system with 5 1/4" cdt audio 2 ways in the back 6.5" Alpine 2 ways in front with rightnow the factory cd player

(need money) but with 2 12" audiobahns sealed boxes with a 1800 watt audiobahn class d amp. the amps only pushing 600 watts right now due to the weaker subs (400w rms each) and i checked my spl out at the school i went to and i hit a 139.00 but thats with NO sounddeadening a normal techno cd (when trying to hit peak invest in a bass cd ) so i think i did pretty good do to the fact my car was rattling like crazy and i didn't have a better source unit
heres the link to the Brown bread and B-Quiet sound deadeninghttp://
www.b-quiet.com/brownbread.html