Powered Subwoofer hook-up

Vehicle: 2005 Mazda 3Door Hatch.
I will be putting a Kenwood WooX powered subwoofer under the front passenger seat, it has High and Low level inputs. I will also have a 4-Ch amp under the drivers seat to power all four door speakers. I'm sticking with the factory HU with integrated Navigation and Sirius Sat. With this configuration I have a couple of questions:
1. Door Speaker Hook-Up: Should I purchase a seperate LOC?, or just use the LOC built into the Amp?
2. Powered Kenwood Woox Sub: Should I get the input by splicing into the Factory HU high level speaker wire? Or can, and should I use the RCA low level output from the Amp?
3. Speaker Purchase: I think I will be getting Infinity REF6000CS 6-1/2" Two-way component system for the front, and REF6002SI 6-1/2" Two-way shallow mount for the rear. Open for other suggestions though?
4. Amp purcase: I don't listen to really loud music but want more than the stock system. Would a small Alpine 4-ch amp be enough? Plus it needs to fit under the drivers seat.
Thanks in advance?
 
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