If you take your time and install properly (sealing off with deadener is fine for the small amount of air pressure these small subs will produce), you can achieve the goal you want. Pair your supporting equipment carefully and tune properly. 80% of making this setup sound good will be in the crossover points and the tuning.

For all that I have heard in the 6-8" sub market, you are going to be hard pressed to find a better performing small sub than the CSS SDX7. And at only $100 ea, they are a steal. For a response down to 35hz, they want 0.5 cubic foot sealed enclosure, which I'm pretty sure you can get out of each stock rear location......again 3 layers of dampening will be sufficient.

http://www.creativesound.ca/details.php?model=SDX7
http://www.funkywaves.net/catalog.cfm?item=css%20sdx7


A review of the SDX7:
http://www.audiojunkies.com/blog/890...r-the-extremis


Because I'm bored I thought I would do some more research for you, of other drivers I might consider:
CDT M6i - $159 ea
http://www.woofersetc.com/p4850/M6i-...ndividual).htm

eD 3.6 6" sub - $85 ea
http://www.edesignaudio.com/product_...products_id=33

JL 6w0 - $100 ea
http://www.woofersetc.com/p2073/6W0-...-Subwoofer.htm

 

to those who have installed these, do they rattle the black plastic that covers all of that area?

 

The plastic area in my car doesn't rattle anymore, I think because of all of the deadening I put inside the two spaces, as well as on top of, and around the entire metal frame behind the seats.

 

my problem is even if these sound good, wont this be running into the same problem that the factory bose sub had, as far as firing forward, and into the back of the seat? from what i understand the reasons why the subwoofers always face the back of the car is so it adds to the distance the bass has to travel from the speaker to your ear, letting the distance between the airwaves "mature" if you will... thats my way of understanding it?

 

I don't know about the physics of all of this (but was told this was the case when mounting them) but as far as my unrefined ears go, they sound fine... The location of the speaker holes vs. the sub position is that there is a little more 'open space' then firing right into the back of the seat. I imagine that this might help a little bit but I could be totally wrong. Oh, and I had a base/non-bose so I didn't really have anything to compare it to.

 

I tried to put a 6.5 JL Audio in there and it rattled against the stock speaker grill. Not to mention all the other rattles. It didn't produce any bass either. I ended up removing it and getting an 8" JL Audio in a custom fiberglass enclosure. It sits in the corner of the trunk. You get good reflection off the hatch glass.

 

i think ill be doing 2 8's in the zenclosure understrut box