lot of great information guys... thanks for all the quick responses... I like the idea on the 8s, because that would also reduce the size of the box...

or u could put the 8s in the cabin, they would get a little louder and it would even be easier to get them to sound good, but the setup wouldnt look as neat as having all 3 subs in one box, but for the most part everyone else here is steering u in the right direction and this would look neat, especially with some neons in the box

put the 8'* in the doors (if possible) or make some kick panels

8'* would be great for midbass

exactly or u could put them in the center councel under the radio and climate control part, thats what im probly gunna do when i redo my stereo

Ugh you're gonna subject your dash to that kinda vibrations? They'll be plenty loud in the trunk, they would be playing in their main frequency area. If you plan to use ported enclosures like you have here, you need to have them tuned well. The 8" enlosuers may be too big on that sheet.

8" kick panels would be kinda HUGE but do-able.

i have had diffrent six=ze subs before. the man thing is the crossovers. most mono sub ams have a subsonic filer that is the same as a crossover but does the oppisite. they act as a high pass crossover that in effect stop the sub from playing below a certian freq. then you use the normal crossover as a low pass(in my case it is also in the amp) so your sub only sees the freq between the two cut off points. this is very important when you mix subs. here is why. the 15 will play way lower then the 10'*. so here is what i would do if i were to do it agian.
cross main subs at 120 with subsonic cut off at say 70-80 hz
then low pass the 15 at same point as above with no subsonic filter or one set very low
yes you will need mutiple amps and crossovers. i put 2 12'* and 2 10'* in my splash ranger and it did okay but it was alot of work to get the sound right. it did not help that i had 2 6 1/2'* and 2 tweeters in each door. never agian i said till i got my trans am were i put a 6 1/2 componet set in the door and a 5 1/4 componet set in the kick panels. whenyou put multiple size drivers you creat the problem of over lapping the the signals and you get spikes in certian frequencies because the drvers reproduce them diffrently. so when you try to use one eq or crossover the drivers react diffrently to the changes creating a new problem. have fun

there should be a way around the vibrations u would get from the vibrations in the dash, besides that, the dashes are fairly solid, as far as the midbass goes, if u have dedicated midbass drivers, shouldnt they be able to go higher than 120, i have also heard of people using them for full range speakers, but im not sure how well that would work

i am currently tring t figure a way to put 8'* in the doors of a excalade. this has proven to be a major hassel but i want to be able to fold down the midgate. the major problem is structual. i would do this before i stuck them in the dash, no offense but that would cause major imaging problem i would think. also it would be hard to create a enclosure in the dash.

i thought the rule was that the drivers should be no more than half of the crossover wavelength away from each other to maintain good imaging, i havent done the math but would that be too far? what about some kickpods?. if not there than what about on the bottom of the dash, would there be enough room, would that be too far, these are all questions that i would of had to figure out anyway

if you can get 8'* in kick pods that is ideal. but i do not htink that is gonna happen unless you give up major footwell room. i would put the 8'* in the door and then do kick panels for the mids and tweets if this is a must. the door will need reinforced to handle the power of a 8" sub but it would sound killer. all the low signal coming from the rear has always been a major problem in car audio. putting subs up front would illiminate that.