hu built in amp question and sub recommendation

[extrarice311]

 im finally upgrading my stock head unit ( 2013 honda city) to pioneer avh 1550. currently i have jbl seps at front and jbl coaxials at back being powered by jbl 640 watts 4 ch amp...since im going to get a HU that has bulit in amp, im planning on having the HU power 1 pair of the speakers ( front or back?) then ill have channel 1 and 2 for the other pair of speakers and have channel 3 and 4 bridged for a 10-12 inch sub...does this make sense? which is better powered by the amp ( and the HU) the front or the back speakers or does it matter at all...finally for my subwoofer issue im not planning on joining any competitions or need a booming setup. just that needed punch to complete the sound experience. should i get a 10 or 12 inch sub, single or dual voice coill? sealed or ported enclosure? please bear in mind the limitation of the amp to be used to power the sub since im not open to purchase another amp just to power a subwoofer...

[DTNS]

use your amp to power your front speakers and your sub, then use your HU to power your rear speakers. (this is the setup I have in my car. )

about the sub, it all depends on how much trunk space you're willing to sacrifice and how much bass you want. ported enclosures usually make more decibels, while sealed subs make clean tight bass. I went with the most compact ported box by Pioneer, the TS-WX303.

Good luck with your sound setup.