Meh, Alpine X and W7 I'll pass for SQ. Do you want real SQ or something that is not boomy, those are two different things. There's so many brands to go with...DLS, ARC Audio, ID....

Look into the DIYMA 12, just google it. It's a good sub and very reasonable in price. TC Sounds/Audiopulse makes some good drivers as well.

It's not always the sub either. IMHO, it's the enclosure/install itself that matters more than the sub. Changing the enclosure specs will greatly change the way a speaker sounds.