The rear soundstage is useless, I would forget about speakers back there and focus on the front only. Unless of course you're carrying passengers a lot, haha.



A nice set of components will sound better, IMO, than a 6.5" coaxial and the 4" speakers. Get 6.5" components and run the 4 speakers (2 tweeters/2 midbass) off the 4 channel amp.



As for the subs, the box you want will depend on the type of sound you want:



Ported = my fav...loud but accurate if the box is designed wrt the manufacturer's specs.



Sealed = slanted towards SQ, but does not have the low freq. extension of the ported design.