right lets get this straight
for the best sound in a car you need
a bloody good set of componenets up front - personally IMHO for the price of those kenwoods you can get a much much better set - genesis, focal, jl audio, phase linear,
an amp to power the front comps, amp should be ever so slightly higher powered than the comps, this way you will not strain the amp, just dont set the gains too high
a sub for the rear, personally prefer either 8" or 10", IMHO you need punchy bass, just personal preference - again, you will have a very wide choice of subs available
decent amp to power the sub, again with a power figure just higher than the sub
and a top notch head unit - with nakamichi, alpine or pioneer (in that order)
essentially this is all you need for a very well balanced system, if you want you can install a set of co-axial speakers in the rear for the rear passengers, though i have disconnected mine, again personal preference
if i was being honest - for the money you will be paying for the kenwood set-up i could find a system that will sound far better