Well if you want a REALLY good sub that would replace BOTH with just one and would be cheap(at least for what you get).....then go to
Elemental Designs The A series subs (from what I've heard from reviews) is comparable to a JL Audio W7. Not quite as good in SQ but for the price.....who cares!
Also it will get louder than the W7. If you have the space build the ported enclosure to their specs.
