Blazer S-10 sub size?

I am currently looking for some subs for my 88 blazer s-10. It isn't that big of an SUV and currently I have 2 rockford fosgate 6x9 and 2 kenwood I think 4x6 maybe, something small like that being powered by a BIG amp for them. A rockford fosgate 900 watt. The local stereo place has a deal going on for 2 12" Kicker comp subs and a smaller amp. I was wondering if that would be to much bass, like i want it to sound good but not THUMP all the time. And i was wondering if i would move the 900 watt amp to the subs and the littler amp to the speakers if that would be lets say kosher?

where are you located?

located near toledo ohio (close to the michigan border)

oh, nm