I left my factory sub closure in place and just replaced the cheap paper imitation alpine sub with a (shallow mount) 8" pioneer --- Much cleaner bass now - This is all I was looking for.

I also replaced the corner dash speakers with JBL (Club Series) & Rear doors with JBL 6x9 (Club Series)
I was told the factory 8.4 head unit does not send highs to the front door speakers due the reason they are just a mid-bass speaker - Adding a 6x9 2way or 3way would be kind of worthless - To be honest I can't even hear my front doors - The rears sound amazingly Clear & Clean!
(I also looked @ Kicker but thought the JBL & Infinity (Harmon & Kardon) just flat out sounded Cleaner/Crisper/Clearer - Just a better quality sound).

Maybe adding a component cross over system in the doors would be the ideal install & also amp them!
Front doors have to have a shallow mount or use a adapter.

Anyway - I was pleased the way my system sounds now - Much Cleaner & Crisp Mids and Highs compared to the dull / muddy stock alpine. I don't even use the surround - Seemed to dull the sound more!
I looked at JBL and Infinity and went with the JBL.

It seemed adjusting the fade - 2 clicks back to the rear - made the front dash speakers not so over bearing. (For My Ears - I found the happy spot)

One of these summer days - I might replace the center dash speaker with another JBL (Club Series) and Keep the other for a souvenir.

Anyway I am rambling so I will shut the Heck up! Look into JBL or Infinity for a budget minded install.

Edit: This might have not been the correct or right way to go about adding Cleaner/Crisper/Richer/More Clearer Sound! (But even the Wife thinks it sounds so much better)!
Mind You - My 8.4 sound level never goes higher then 15 - this is mostly when going down the interstate @ 75-85mph.