Actually with the way the speakers are facing, perpendicular to the rears and opposite the fronts, it doesn't help with volume very much , due to some cancellation in sound waves
It isn't too terribly bad, considering it's behind the seats so cancellation with the fronts isnt too bad since the sound waves bounce off the walls of the car before it comes into contact with those two etc.
Try it out for yourself
those are actually two 6.5s in a sealed box, now vented for a bit more volume
(they do not share the same air space)
i put the tweeters for them as well as the type X speakers in the rear panels like can be seen in the pictures

 

Thanks for all the comments guys

Yes I definately plan on carpeting it its just I finished it like 1 day before I had to come back up to school and right after I got my wisdom teeth out. So the carpet has to wait for now. Yeah in reply to chrisfun8806, the reason I got alpine was I had heard pretty much nothing but good things about them. All the stuff sounds amazing, im not a huge audiophile or anything, but I think its sounds really good. And I still have to finish tuning all the gain stuff and adjusting everything. As far as the amp, how much can you get that one your telling me for?

Here's what I spent on everything (well I got a few things for Christmas but)

Oh and I correct myself on the head unit, I got DEH-690UB not the DEH-P6900UB (even though there's very little difference). Anyways, I got all my stuff from onlinecarstereo.com and got great prices compared to the usual.

Pioneer DEH-690UB: $215
Alpine SPR-17C: $87
Alpine SPR-69C: $98
Alpine MRP-M650: $193
Alpine MRP-F550: $225
Alpine SWR-1242D: $139.50

Plus like $60 for the 2 amp wiring kits, and 4 channel and 2 channel 6m RCA cables from knuconceptz.com, and like $40 for the MDF so I spent like $1100

 

good prices....lol if u want an alpine headunit to go along with all that, im selling my cda-9855.

 

I chose the pioneer head unit cause my friend had it and I loved the ipod interface and the display.

 

gotta love Alpine i bet that sounds super nice

 

eventualy i want to upgrade my interior speakers to all alpine componets, just to cheap right now...
expetialy with puting my one civic back stock


but for you guys thinking of just puting the sub in the back seat... it wont be good at all... keep the subs in the trunk for your best bass reflection off the back of the trunk lid...

 

hey itekenbo, i got my amp for about $250AUD, however its second hand though. I thought I got a good deal at the time but now looking at your parts and prices...I'm not so sure anymore...

 

$250 AUD is about $220 USD so yeah you could have gotten a bigger amp like mine for the same price. You should really go to onlinecarstereo.com

 

or ebay :P

 

Yeah ebay too except you don't always get the warranties then though.