Hmm, with the 3rd row it is difficult. You'll have to go with a simpler setup. Maybe use a passive crossover setup rather than an active like mine (so you wouldn't need the Audison Bit One and you don't need multiple amps.)
Here is something you can do. Use a multi-channel amp to power the components that you put in today as well as a subwoofer. Here's a JL 5 channel amp.
http://www.jlaudio.com/hd900-5-car-a...plifiers-98223
This is the sub I have, but you can choose a smaller one if space is an issue. You really just need 1 sub. I recommend JL's sealed pre-made boxes. I've tried many subs and these are the best:
http://www.jlaudio.com/cls110rg-w7ae...-systems-93270
http://www.jlaudio.com/car-audio-sub...stems-prowedge
So get an amp and a sub and that's pretty much all you need. If you still want the KRX2 later you can add it, but that's an expensive upgrade and the $ vs utility is going to be less than the initial amp/sub upgrade. Most people are happy after an amp/sub upgrade.
As far as mounting the amps, it'll most likely be behind your 2nd or 3rd row. Something like this:
