Well to update...I returned the Infinity Kappas back to Frys and got my $190 back. I borrowed a few subwoofers from friends and am actually getting what I would consider to be really good sound.
But since I am borrowing them, I will need to give them back...
I borrowed some Sony Xplod 10's from a friend and those were not what I am looking for. I also borrowed a JL 12" W0-4 which sounds pretty good. It is only a 125watt speaker and is mounted in a truck box, but it sounds pretty good the way I have it setup. The car does not have much rattle at all and since it already has a decent amount of sound insulation being a Lincoln, it is not very loud outside of the car either (a huge + for me).
My amp can either do 2x100watts @4ohms, or 300x1 @4ohms...I have the amplifier running the JL in bridged mode right now so I am not listening at very loud levels since it is twice the recommended power.
So here is what I am looking for:
A single subwoofer that can handle 300watts @ 4 ohms bridged from the amp that is in there.
Any recommendations? I don't really want to spend too much money as this current setup sounds pretty good to me...just need to make it mine. I might not even replace the rear 5x7 speakers.