Need that punchy midbass

Well my buddy is getting a 2001 hyundai accent with 10k miles for free so were movin his system outta his saturn and into that beast.  So far hes got an Alpine 9811 deck, Alpine 6 1/2 type R componets in the front and the rear and 2 12" mtx 4000 in the back.  Running the 2 sets of components off an Alpine amp and the subs of a Rockwood(lol) not sure of the exact wattage but it is sufficient.  Hes got the 12s in a fairly "large for their specs" ported box.  They rumble pretty damn loud for the cheap subs and the cheap rockwood amp hes running em with.  He wans to keep the box and sub setup cause he likes how low and deep they hit but wants more mid-bass.  We were thinking about running 2 8" drivers, crossing the 12s over to 70-80 hz and letting the 8s take care of the 80-250ish hz.  Looking for opinions on this idea, results with anyone whos tried it, and also how we would work the cross over for the 8s so they only cover the 80-250ish range.  Also is that a nice cut off range for those 8"s, like i said he has 2 sets of 6 1/2s.  He listens to a lot of techno, so likes that "getting punched in the throat" mid-bass.

Thanx for any help.

Yes, you can use a set of 8's or 10's for this application.  I use a set of 7" woofers for this in my car (Scanspeak.)  I suggest you want to match the timbre and sound quality of your main speakers, meaning if the mains are 6.5" from a particular company it would be good to get a set of 8's or 10's from this same company so the sound quality matches.  Getting a nice blend will require some time in the setup and a very good crossover.  Look at Audio Control 3-way electronic crosovers.

An MTX sub driven off a Rockwood amp is like a decent car being pulled by a donkey.  It might work if that's all you have available, but it is one crappy way to go.  Rockwood is my definition of POS.