you dont have a ton of power going yet so more batts isnt a huge deal for you. a good idea to get another though. my 1st setup was 2 10" polks, sealed, on 250w a piece. that system impressed me and my friends back then. now im at 145db, 3500w and i want to get it louder. as you get used to the volume you will want to go bigger and bigger. oh, and your friends wont be "impressed" anymore... they will think you are crazy.
ill throw a number out my ass for you... 138.5.
lol pm me after you get it metered. i want to know if im close.