What stereo upgrade next?

Ok here is my current setup:

Alpine SPR-17Ls components in front cabin, Kenwood KDC-MP828 hu, Usx-2050 50 x 2 Rms amp running components, Oem infinity rear door speakers which I believe are running at 8 ohm providing bass and rear fill.

I have two questions:  1.  What would be your next upgrade, looking to get a little better bass.  Subwoofer or new rear doors speakers?

2.  How much of a difference would a larger amp make in the sound of the front stage say if I bumped up to 75 x 2 or even 100 x 2?

1) subwoofer

2) A bit more headroom, no real difference in SQ.

So upping the amp in the front is going to make it louder but I'm assuming the bass reproduction is not really going to improve?

I was debating adding another amp.  If I bridge the usx 2050  I get 160 x 1 at 4 ohm.  Will this be adequate to run a sub?  I don't want anything really ghetto but some decent bass.  Also how will that work with the comp's that I have running in the front.  Currently I am using the crossovers from the components not the amp settings.  Will I be able to redirect the base away from the front and to the rear only with a new amp.  I'm assuming that I will hook the 2nd amp into the first via the rca's.

A subwoofer would be a better upgrade than rear door speakers if you want more bass. A single 10 or 12 inch sub would probobly work well.

This inexpensive combo would give you some good bass.

http://www.cardomain.com/item/INFREF1232W

http://www.cardomain.com/item/MTX282

Ok I have decided to add a sub to my recently upgraded system.  I currently have Kenwood kpc-mp828 with Alpine SPR-17Ls components front powered by usx-2050 50 x 2rms amp.  Rear doors are stock infinity 6.5's running at 8 ohm( I beliveve) off of the hu.  Vehicle is 2002 jeep liberty.    I am looking at various subs but don't really know what I need.  Want decent bass not wndow shattering.  Physical size is an issue.  Not much room in the rear compartment and I do have to carry things around occasionaly.    Can anyone explain voice coils and ohm ratings for subs?  I see some are 8 ohm, 4ohm 2 ohm, single, double voice coils?  I think I am leaning twoards a bazooka tube just based on the tubular shape and how it will fit the vehicle.  What is the sound difference in an 6" or 8" or 10" sub.  Also how much wattage rms is needed to have decent output?  150, 300?

Will be adding a second amp to drive the sub?

Ok lets say I go with this model.  I currently have my usx 2050 amp hooked up to the front Rca output on my hu.  Speaker wire running from the amp into the crossovers for my alpine components.  When I add this package Am I going to hook this to the sub output on the hu or the first amp?  Currently my first amp is on amp setting with just the gains set.  The high pass and low are not adjusted because I assume the crossovers are doing that.

I'm assuming that I want the highs coming form the tweetes, the mids from the 6.5's in the door and all the bass coming from the sub?  I would prefer to only run into the amp for ease of installation.