could use some opinions, altima stereo

I just picked up a nissan Altima to use as my daily driver since it gets about 3 times the MPG as my Mach 1 does. I wanted a car to just get in and go that is left completely stock that I can park anywhere, valet anywhere without worrying as much as I do with my Mach. Well I now have about a 1 hour commute to work each way and the stock stereo is really bugging me. I do not want to change out the head unit and I do not want to spend gobbs of money on replacing all of the speakers, running multiple amps ect. Here is my plan and I was hoping to get some outside imput on it because to me is sounds like a solid plan but who knows.

I don't want to loose any trunk space but I need a sub of some sort. On the passenger sidewall of the trunk there is enough room to build a fiberglass box to house a small sub. I am planning on using a Tang Band W8-740P 8" sub, they do well in small enclosures and at under $70.00 it is hard to beat. For the amp footprint is a major concerne. I do not want the amp mounted to the back of the rear seat or build any additional panels in the trunk to house it there. Under the front seats there really is not enough room and with the airbags in the seats I would prefer to not have to pull one out. There is enough room to mount a small amp under the rear deck so thats my plan for now. About the only thing I could find to fit would be one of the Arc audio minis but the price tag on that is a bit high so I think I am going to run a JL XD 400/4. It fits the size, has a remote level control and should put out enough power running ch. 3&4 bridged to the sub and ch. 1 & 2 stereo to the front.

I am not worried about rear speakers but I do plan on changing out the front stage. This is where I am second guessing myself. I am considering dumping a 6" woffer in the factory door location and a pair of 3 1/2" two ways in the OEM dash locations. I had considered doing a complete 3 way seperate system but I do not want to have to figure out where to mount the tweeter. I am thinking I would run the HP filter on the amp and use a passive crossover network from there. For example with the HP filter on the amp and then do a passive LP filter on the 6" woofer I could effectively make it a bandpass. then do an additiona passive HP on the 3.5" speakers in the dash. I realize that it would not be as good as a true 3 way system but it has to be better than just replacing the door speakers with 2 or 3 way speakers right?

Now I am just trying to figure out crossover points to use. the 6" I was looking at is good to about 3k so I figured I would cut it off at about 2700 or so and let the 3.5 get everything from about 120 and up. Is it ok to have that much overlap between the 6 and the 3.5" should one stop right where the other picks up?

Using a 6.5" subwoofer in the rear door locations can be very effective as long as you don't expect the big thump of larger diaphragms.  I use a set of Tang Band 6.5" subs from Parts Express, driven by 100 watts each they sound pretty good and reinforce the low bass from my front components.  Give it a try.

Also, your crossover point from a woofer to a tweeter is determined by the tweeter.  What is the fs of the tweeter?  You generally want to cross over no lower than 2X the fs.

Well the tweeter and the 3.5" woofers in the dash will already have a x-over built in. I am wondering what X-over point I should use between the 6" woofers I am putting in the front doors and the 3.5" 2 ways in the dash.

Basicly I plan on setting the HP on the amp at about 80 then using a LP X-over on the door woofer to make it a bandpass then putting a passive HP filter on the 3.5's. I just can't decide where I should cut the door woofers off and what HP fz I should use for the dash speakers.

So it would be like this

8" sub LP 100hz

6" door woofers Bandpass from 80hz to 2500hz

3.5" 2 way dash speakers HP 150hz

I am just not sure if the 2500hz on the 6" is too high and if the 150hz on the 3.5's is too high or low.

Well I have started on getting things squared away on this. So far I have ordered some Boston Acoustic S35 3.5" 2 way speakers to put in the dash. They have a frequency response of 100Hz - 20kHz. I had thought about building my own passive for it but after browsing parts express I can get a pre-built unit for less than I could build one so I figure I might as well try it that way. The crossover will be 100Hz 12db per octave. I also ordered an amp, I have a mint PPI Art series A404.2 that I was going to use but trying to keep things as compact as possible I decided to try and find something more compact. After a long search I came across a micro amp, Vibe Audio Litebox 4 rated at 90x4 and only 5.25"x11.75" I can easily mount it under the rear deck plus it has x-overs built in which the PPI does not have so it fit the bill perfectly. For now I am going to just drop the 3.5's with the x-over into the dash and start building the fiberglass box for the 8" sub. For the 6" door speakers I am going to use some Tang Band W6-789E's which are 8ohm have a response from 50-10kHz and their sensitivity is 92db which matches the sensitivity on the Boston 3.5's.

Now I am starting to wonder if I should just ditch the passive x-over and run the 3.5's and the 6.5's straight and just use the HP filter on the amp set at about 100hz and call it a day. Since the 6.5's resonse is 50-10kHz will I start getting harshness out of it from any thing above the 10kHz? What am I giving up by filtering them at 100hz rather than letting them go down to 75 or 80hz? I am not trying to build any kind of competition system in any way but I would like to get the best possible SQ out of it.