Subwoofer help: Cambridge Soundworks vs SVS vs custom

[Nemesis]

I need help choosing a subwoofer. As I have said before the plan was for me and Kyle to build a sub in the near future. Then I found the SVS PB10. Then I Kyle's Cambridge Soundworks Newton Series™ P500 Powered Subwoofer. I know that the Newton is in another price and build class than the PB10, but the SVS seems to have the raw numbers. Since SVS and Cambridge (own stores) sell directly to consumers, doesn't that mean that they can charge less than a company that doesn't. I'm confused and anoyed that I can't go to a store to listen to the SVS. Oh the Newton fires in both directions and periodically goes on sale for ~$500. I'd buy in store and not have to worry about shipping (bu then there's our lovely Mass sales tax...).

Also would it be possible/practical to build a sub that aproximates the capapabilities/numbers/ sound quality of either of these subs? Sounds pretty hard... Both seem pretty good.

Links

Cambridge Soundworks Newton (http://www.cambridgesoundworks.com/store/category.cgi?category=spk_subwoofer&item=c1p500zzz)

SVS PB10 (http://www.svsubwoofers.com/subs_pb10_isd.htm)

[Mjr7531]

Don't look at having the dual drivers as an advantage for "firing both directions" rather it makes the sub 3 dB more sensitive and a little harder to place in the room.
If you like a good challange and not have to worry about markup, go DIY, it's fun and rewarding.