i say JL all the way.
they are more expensive, but u get what u pay for
the sound quality is better than anything ive heard. u dont get that hollow echo sounding beat when it gets real low, even in my truck w. the subs right behind me.
and JL amps give u the actual wattage they say they give u, unlike every other company who says "2000 MAX Power" which mean they somehow squeezed 2000 watts out of it in a lab somewhere and its impossible for u to get more than like 300-400 out of it.
u dont need a new head unit, but im sure u will need some converter to get ur amp hooked up, but hooking up car audio is easy.
its a battery cable, rca's from ur head to the amp, ground, and speaker wire.

it can get a little more complicated if you have a newer car then u have to get the box that enables ur car to still make the ding sound when ur seat belt isnt fastened or the doors are open with the key in the ignition.

another upside to JL is that you can get 8" subs if you want. iknow ur not wanting alot of bass, but an 8" W7 beats pretty hard. harder than most people sets of 10s.
and the diff between 10s and 12s is pretty big
i had 2 10" W0s and upgrade to 2 12" W1v2 and it was an amazing difference
now i have 2 12" W3v3 and they are ridiculus

but if your not wanting ridiculously loud bass, i would just go with 1 10" to save space.
but im sure once u pull up on someone else with blaring loud music, ur gonna wish u would just went all out and got somethin to out beat people with.

also, the best place i found for car audio online is onlinecarstereo.com
they have GREAT deals, like alot of deals that are around 50% off of when u would pay in any store
my gf got a 500/1 TMA amp (which i would also recommend, they are a sub-company of JL but still have great quality for half the price)
for like 160 bucks. theyre usually 250 or 260 in stores.
same thing with my W3v3's i think theyre like 160 a peice and like 300 in stores

 

Though I'dhelp clarify a few things real quick. With the stock system 1. All your speakers are powered by the stock radio's amlifier (no need for an amp for the reg. speakers). 2. Yes you can keep your stock radio and still have a sub. You will need the LOC thingy to get a useable signal for the subwoffer's amplifier. All it does is take the bass part of the signal sent to your speakers, and transform it to the signal the sub's amp needs to make bass.

As for the wiring, all you need is a few things. If you get the infinity deal, all you'll need is a power wire (like 8ga. or so, run from batteryto amp) a short ground wire (at least 8ga. or larger, find a bolt in your trunk to stick it under) an RCA cable to run from the LOC device to the amp, and a remote wire (turns amp on and off with ignition, can be wired to any switched 12V source --> any 12V source that turns on and off with your ignition key)

Like someone else said, wal-mart sells amp kits for like $20-$30. Used it in my truck for like 4 years b4 iupgraded. Get one, it has everything you need to wire your car for subs. BUT you will have to get the LOC thing somewhere else. Can't remember where my buddy got his.

Lastly, the nice thing about using the stock system (besides nobody wanting to steal it) is that you can turn the bass up on the sub just by turning the bass up on the stock stereo. But itWILL turn the bass up on the other speakers, so be careful. Since the infinity has its own bass ****, you'll have two ways to turn it up or down!!

Good Luck with everything!! you should be able to do it for pretty cheap. I'm thinkin bout doin the same type of set up for my gf's ranger.

 

i have a nice simple setup in my mustang it hits pretty good:

Alpine CDA-9856 cd player (i only use ipod input tho)
Alpine Amp
JL 10W3 V2 subwoofer.

not deafening bass, but a nice proportionate amt.

 

I drive a 98 Camaro, thanks Crash, that does look good, it'd fit perfectly out of the way.

I actually impulsively bought a pioneer 12 in 800 watt max sub last night at wal-mart, but I think I'm gonna take it back. My brother was a carpenter, so I was thinking of just making my own box to fit neatly in the hole where the t-tops go, in the trunk and putthe the 12 in in there, and mount the amp to the top of the box right beside it. Since thats the farthest back and deepest the trunk will go, I'll have enough cord to put it up on the little area behind the seat while driving, but then when I stop somewhere I can drop it in the little t-top slot and nobody will be the wiser. Thefts are insane around here.

Either that or just get the MTX you mentioned or the Ininity unit. I'm really strongly leaning towards the Infinity since I found somewhere I can possibly get a new one (used for a year) for less than 150 bucks.