Can I get my JL 12w7 to hit alot harder if I build a bandpass enclosure for it or will the H.O box beat it?

Bandpass enclosures aren't really loudness-based enclosures...they're for people who want a specific narrow-band of frequencies to be reproduced (such as 20hz-50hz). If you want to boost the loudness your best bet would likely be a ported enclosure tuned to around 50HZ...BUT you will get the overall best sound quality from a sealed enclosure. Also, your amp or headunit could be the issue. Make sure you've got a good amp (I would expect to pay AT LEAST $400 to get a good amp for that sub; and make sure your headunit has decent output voltage (4V or more).