12" sub box, any advice for coupe box design? Options

[klr_toy]

ok I bought a 12" Kenwood W3003 sub in a big ported box (for two 12's) that barely fits in my trunk. First off i don't know much about Kenwood, anybody know how good the sub is? Right now it's bridged to a Profile California 240 watt amp and sounds decent. It's rated for 600 watts peak though so i know it would sound a lot better with something bigger. I'm planning on keeping the 12 and buying another 12" and a bigger amp to run them both, and building a new unported box to put them in. A couple more questions... Is the W3003 i have worth keeping, and if so what is another good kenwood 12" to pair it with. or should i just sell the Kenwood and get 2 new subs. I will be building the size of the box to the speakers specs, but was wondering what other people on here have done. I'm especially interested in coupe installs. i saw a really nice coupe setup on here the other day. um ya thats about it. I'm open to any advice/comments. Thanks