Ascendant Audio, there line is growing.... to meet your criteria they only make two 12" models and are developing new non XBl^2 technology subs.
There is no such thing as lower quality, but the cheaper priced 12 is the ATLAS and the more expensive with more XMAX and XMech is the AVALANCHE. These are SQL subs that don't distort and low end monsters.
http://www.ascendantaudio.com/