kicker sub matching 20hms

i need help choosing a sub/subs dvc 2ohms for a kicker kx2500.1 mono amp im so confused after trying 15" l7's 15"l5's and my old school kicker comp vrs 12" are still louder i dont get it any more im lost http://www.ebay.com/urw/Kicker-KX2500-1-1-Channel-Car-Amp/product-reviews/62458550...