if you're wanting in cab, maybe look at PartsExpress Goldwood dual voicecoil 8" woofer. you might get a lot more from stock audio power with both 8ohm voice coils connected parallel. another decent option for a budget is an MCM 8" http://www.mcmelectronics.com/product/55-2950 does about 94db on one watt. want deeper thud? http://www.mcmelectronics.com/product/55-2421?green=D353EADF-849D-5992-BE6F-E33554E0AA2E . an 8 that'll reach down to 25hz? hey now

not kidding about "just stock power", if you drop a high efficiency driver it might surprise you a lot. another possible help is a freeware program called "box-notes". enter the dimensions of the cabinet and size-specs of the driver, it'll calculate the diameter and length of port tube's tuning for you is pretty darn nice. I'd just used the program recently to build clones of "the Clue" stereo speakers, the 2 6.5" woofers DO give the impression of a subwoofer running, from 6w input power?!