I'm trying to decide what speaker will complement my system best.

Here are my requirements:
1)To power Im using a Kicker 650.1 (claims to be underrated and produces 745 actual watts @ 2ohms)
2)A single 12" driver, DVC 4ohms
3)600 watts rms handling (or close to)
4)$200-250ish (will go higher if it needs to be done)
5)Will be in a vented box to spec
6) 65% SQ VS 35% SPL

looking at MA audio's HK120XC but Im still in the research stage.
Any suggestions on other woofers?

 

Couple of options for ya, from what I deal with in your price range.

RD Classic 12" 2x4ohms 500wrms
American Bass XD12
Clarion PXW 12"

All are under $200 brand new, and sound great. The AB and RD get a little louder then the Clarion IMO. All are great products.