he's got a sub already.

1. for coaxils, rear ones at that, 2 way is fine. anymore is marketing gimmicks. you dont need 'super' tweeters or an extra tiny mid-range that does nothing.

2. depends on how loud you'll be playing your system.. theres plenty of power in that amp to get all speakers loud but theres also potential for clipping..

whether that amp is 'adequate for your needs' depends, again, on your expectations and how you'll be playing the system.. but for the majority of people that amp will be fine for most applications.

is it a daily car??

personally id get a 2chan for your fronts, say the t400-2. more power to the fronts. id rather have my front splits be used to their full potential than compromise that for rears and less power all-around. and heck its not like the rears wont go loud off the HU once u HP them.. does your headunit have x-over filters??

or t400-4 used actively..

im done.