i would say it depends on how good the 15 is. If you want somethin louder why not just remake a box for your 12? If you only want to spend 100 bucks on a sub then i wouldnt really say you are gunna find anythin much louder then your 12"
Well, i have 933watts going to it, and it is ok, but not as loud as i expected. I wish i could find another RLP for cheap, but this is one of the older ones.