You will need a JL cleansweep or some other kind of
processor or amp/processor to flatten out the head unit
output.
If you haven't bought anything yet I would get in touch with these
guys for a a one stop shopping solution.
http://www.musicarnw.com/page-2/
They can sell you a kit with everything needed that plugs into the factory connectors. It comes all setup so no soldering or crimping and little if any
adjusting is needed.
If you really don't want to replace the underseats I think you might be OK
with just the amp and JL Cleansweep. If you want fantastic bass you should
replace the underseats though.