I like the concept. Most people think you have to pay big money to have a good sounding system, That's defiantly not the case. I would like to help out, if you don't mind me offering suggestions.
I'm into audio big time and the less i have to spend on it the better.
Here is my opinion, take it or leave it.
1) focus on the front stage. if your going to go subless your going to need large speakers, forget the rears. The doors will need deadend and sealed. shoehorn the largest mids you can find. put reaching 50hz at the back of your mind unless you like to listen at low volume. make 80hz or higher your goal. Your going to have to rely on your hearing's ability to fill in the low notes, its an odd thing but its real. cabin gain helps this too. you're going to need plenty of power for the fronts. I would run around 100w per channel.
2) Allocate most of your money towards the front speakers, power for them, and acoustic treatments for them. You absolutely need to plug up the gaps in the door to coax the bass from the mids.
3) Do not use fatmat, it does not stick well and provides very little if any benefit. Use a few square ft of dynamat extreme or second skin. They should be the same price, you may get less sqft but it works much better. you only need about 4 sqft to stop the doors from resonating.
4) if your going to run low cost speakers you need to have sound shaping abilities, this means finding a eq with at many bands as you can get, either featured in the headunit or a cheap external one. A equalizer will make up for any short comings that low cost speakers may or may not have.
If you would like more suggestions, there's more. If not then don't mind me. you've hit on something that i'm very interested in and would really like to help.
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Alpine 9857, JBL MS-8 DSP, US Acoustics 2x80w & 2x100w, Vifa 1" XT25 Dual Ring Radiator Tweeters 3k+ 24db slope, Dayton Audio 7" RS180-4 Mid-woofers 100Hz-3k 24db slope, 500w rms Kenwood Class-D Mono - JBL 10" GTO1014 Sealed. 100Hz- 12db slope