check the TS-W3003D4, they just came out and cost only slighly more than the TS-W310D4.
They have a bigger magnet and a rubber boot, otherwise the cone and surround are the same.
Rated power is higher, 600W rms each
I have 2 of these new ones in my Isuzu Trooper, and have replaced the TS-W310D4.
I do feel they go a bit lower. I have them in an enclosure bigger than recommended, but I get pretty good low end.
It hits MUCH lower, compare to my Polk PSW505 12", which is a "HT" sub.
For around $200 you can get 2 of these new ones and an enclosure....
Also, according to the specs, these should go lower compared to the Infinity (?)
Left is the TS-W310D4 Champion 12", right is the TS-W3003D4 Champion PRO