Here is my current setup :

Kicker 400.2 for my fronts
MTX 4244( 4 X 50w rms) for my rears and 10'' sub ( 2 x50W bridged = about 150 rms)

The mtx sounds no way like the kicker , thats why the Kicker drives the fronts instead of driving the sub. Ok now I want more bass , specially more midbass ( 80 hz to about 200 hz). I want more bass definition ( SQ), specially more punch.

So I was thinking of bying a third amp , something like the Kicker 600.4 ( 75 rms x 4 into 4 ohm) and two 8" cerwin vega IT that I would put into 2 sealed boxes.

Then, the new Kicker would drive the rears and fronts ( 4 X 75) , the other Kicker would drive my 10'' sub that would stay in the trunk. And the MTX would drive the 2x8" subs ( so about 150 w rms going to each sub) that I would place on the top of rear deck ( hatchback car) .
Don't tell me to use the MTX to drive the fronts and rears because I prefer the Kicker ( much more cleaner at high volume )

My questions : Am I doing this right ? Will I get more midbass? Do I really need a third amp? Please help.

I've always heard that mixing different size subs gives cancellation problems. I would say that if you're looking for more bass to either port your sub or just go with a newer 12. I wouldn't waste the money on ordering some 8"'s to add to your system.

Using two 8's to both get more sub bass and more midbass would not be a good idea as you need to use seperate speakers for these frequencies otherwise sq will suffer. You could use the 8's for frequencies 60-250Hz and the sub for <60Hz but then you have all that nice strong midbass coming from the rear. How about adding 6.5's up front to add midbass to the front stage and then add another 10 or use more power or a different box to get more bass.

Running three amps is a lot of wiring. A nice 5 channel would work nicely in this setup. Or a 4 channel and a sub amp.