I do tons of research before i buy my subs, here is what i have found out from experience and research..
I have heard the following:
Rockford HX2's - THE OLD OLD kind, SOUNDS GOOD
Rockford Punch p1's - Sounds like crap
Rockford Punch P2's - still sounds like crap
MTX Thunder 10's - Very tight bass, Sounds good
Kicker CVR 12's - Sounds loud but definitely not clean at all, crap.
Volfenhag - for the price it bangs hard but it is cheaply made and is really just spl, no sound quality.
Alpine Type r - Sounds good but has a different sound to them.
Polk Audio - sounds clean but way over priced...
Phoenix Gold RSDC - Amazing, this sounds extremely crisp and clear and is loud. Far better than a Type R imo.
I will honestly tell you to go with some of the brands you have never heard of, I bought my Phoenix Gold RSDC not knowing exactly what to expect but i have to say that i am amazed at the quality and sound of this sub.
I spent very little and got a huge performance. Get a quality subwoofer, a quality setup and i would recommend a custom box and you will have a sweet setup.
pg, si, daytona are all good choices for low prices.