i have a 92 civic si hatchback and i wanna make a false floor and drop a sub (maybe 2) in the spare tire well. so i want either a good 10" or 8" that doesnt require a whole lot of box space. money isnt too big of a problem. i dont wanna spend 500$ on a sub, but im willing to spend a little extra to get the right sound. i was lookin at the diamond hp10 but i dont know how much space it requires

 

i know you are looking for a 10 or 8 but kenwood makes a shallow mount 12 (and so does about every other company now days but i have never dealt with them) it requires if i remember right a cubic foot of ported air space so you could do a shallow mount that doesnt require much air space. and since i have dealt with the kenwood i know it does sound really super clean i was amazed it being just a kenwood and not the exelon series. and its about 400 rms. it runs about $200 for the sub give or take depending on where you go.
but a shallow mount could be a good way to go just make sure you get the air spaces right they can be tempermental just like any other sub when you dont have the right air requirements.

 

I had this setup in my 94 Si

I had 1 Treo 10" and loved it.

My only complaint with this set up was that it wasn't over powering.
I had MB Quart 5 1/4" + tweets with musicomp crossovers and the system was well balanced over a variety of music. but, you were not going to overwelm any body with a bass CD. If I had done two 10"s I might have achieved that.

this might give you some Ideas

 

I have always been a fan of JL Audio. I currently have (1) 10W3v3 in a small fiberglass enclosure and it sounds incredible. They don't need too much space to perform well.

 

yeah i looked at the shallow mounts but they dont have as big of a frequency range. and i was also looking at the 8w3 and the 10w3

 

forgot to mention itll be powered with a rockford 600w punch amp.