I've finally gotten tired of my pidly stock WRX sound system, and have decided its time to upgrade. I am not an audio geek but I would like to have a good "value/quality" system (as per the faq). I listen to rock ~ 70% and HipHop / rap ~30%

Goals:
Beter clarity, a little more bass just to give it a nice fill(don't need the whole world to hear me though), as little weight as possible (auto-x and track freindly), as stealth as possible, still have a decent amount of $$$ for my addiction to power and handling mods.

Constraints:
#1 Setup MUST retain the stock HU, for stealth reasons, and the fact that there are a lot of stereo theifs around these parts. Plus I have grown accustomed to the 6 disc in dash. #2 Setup can't take up any space in the trunk since I load it up a lot and am not willing to sacrifice anything there. Which makes me very interested in the powered underseat mounting subs. (02-03 wrx seats are lower than a lot of others).

Setup I was thinking of going with:
David Navone NE-774V LOC ~$35
Infinity reference 6.5" 6000cs ~$110 (for the front, nothing for the rear)
? 2-channel amp capable of driving ~90 watts rms to the fronts cleanly ~$?
Kenwood wOOx ~$170 or Clarion SRV303

DIY Upgraded positive and grounds in the engine bay ~$?
DIY value type cable wiring (suggestions?) ~$?
Minimalist raammat install ~$?

Trying to keep total cost around $600ish. Suggestions maybe other comparable options.

I would suggest to forgot the infinity and get CDT CL-61 since they are still the entry-level value to beat on the market IMO. but since you need an amp, and one that can get under a seat, you could go with an Elemental design amp (9.2 or 9.4) and edi6000cs, wich are probably the closest to CL-61 component system you can find in term of price and sound. with a 9.4, you would have something like 50w rms x2 for the frontstage, and 150w rms for a subwoofer. DIY your own box with a low mounting depth 8" sub under a seat would still be the way to go IMO. if you don't want to break the bank, a single dayton quattro 8 is 50$ and would sound good in 0.25 to 0.35 cuft.

I was thinking about the CL-61 from thezeb (since I could swing the $150 for decent components) but I hear they are on backorder there, plus don't they require some extra modifiactions to fit even with the thick large spacer from the vendor here?

As far as DIY underseat box, does anyone have easy to follow directions/dimensions for a box that will fit the 02-03 WRX (lower than others).

Just a note, the underseat sub in the WRX is made by Clarion and is almost identical to the clarion one you mentioned. I talked with one of the Clarion engineers about that a while back, I joked with him that it looked the same, and he said thats cuz it is the same. Its not identical, the driver is different, but for the most part, they are the same. As for the Woox, it doesnt produce much bass, nor does it produce it very loudly, but it does produce relativly deep bass. I used to have a sheet I did measuring this sub setup in my own measuring room setup. My equipment is very crude, and I am yet to create one that can take into acount room gain, other than through simulation. It has a response, without cabin gain, down to about 50-55hz, but the -6db point is close to 35hz, so I think it will probably produce bass down to about 35hz or so. However bass falls off sharply after that, so cabin gain or not, it isnt producing anything lower than that.

It wouldnt suprise me at all if the Dayton Quatro4 coudnt fit under the seat without an enclosure. You have very limited height, and a lot of stuff in the way. In order to get propper clearance you will probably need to use either Fiberglass or composite materials, otherwise I dont think you can get .2 cubic feet of space, including driver displacement. Infact, If I was to go this route, I would be as creative as possible in getting as much space as possible. I would probably use Fiberglass, as its easy to work with, and I would follow the shape of the floor under the seat, then create an enclosure that takes up every nook and cranny going up as hight as the seat height. It would probably involve removing the seat to install, and it would probably still only net you .5 cubic feet. This is my guess though, I did see someone discuss making such a box to sell on these forums, and he gave claims I couldnt believe, but I wouldnt be able to know without building one anyway. Never actually measured the whole area.

Finding a flat woofer such as those from Kenwood, Blaupunkt, Illusion, etc would be a good idea, as it will give you shallow mouting ability. I would go with an 8" shallow mount subwoofer, seems like I see more and more these days anyway.

As for small amps, see how much space you have, and then figure things out, I havnet measured so I cant say. However, the smallest amps I know of that I would also recomend are from Blaupunkt and also arent made anymore. They are the ones that look like Imacs and were actually designed and mostly built by Class-T inventor Tripath. They were off the shelf modules from Tripath, and were far and away better than anything Blaupunkt had ever sold, or has sold since. If you remove the plastic enclosure, they become the smallest amp modules I have ever seen, and they are very efficiant. Find them on Ebay, they are usually very cheap.