Joe X

Your box probably is hitting the car resonant frequency and may be oversized to show muddiness and showing a sharp drop at higher bass. It is interesting that you provide no box specs, you see it doesn't matter much that the enclosure sound normal in another car. Also don't see what is your system, models for the speakers, amps etc. You provide no pictures to document your tests, etc. Even if you have an acoustics problem, there should be a way to work around the issues even if that means try different components, etc.

You can easily run kicker 12s which like 2.25-2.5 cubic feet each, You likely don't have 8 cubic feet net, most trunk cars don't have that much. There are some 15s that will run in lesser airspace. like DC Audio brand which will need 3 cubic feet each but very few 15 subs will run great below that. When you have space limitations you alternative is more power and subs that can handle the power to get loud in smaller boxes, there are subs that can handle many thousands of watts of power each.

Here is a box made to be similar to the one you linked but a bit better for the lows and the cutsheet can be completed just with a table saw and a jigsaw, no need for router: Power: to sub(s) rated, amp subsonic filter to 28Hz, minimum port clearance 4", figure is just an assembly guide:

Those are specified to work in 3 to 5 cubic feet net INTERNAL each, say, we pick 4 cubes for each as kicker subs prefer larger boxes and you would need 8 net, you don't have that. You are calculating external volume, net internal volume is the airspace inside the box, the space occupied by the sub, the port and the wood doesn't count. You will only be able to properly fit 1 sub of those. but it would be louder to use 2 12s that would need 4 to 5 cubic feet net internal that you should have.

It could be bad RCA terminals (either end) or the amp itself, very possible if you installed the amp in the sub box. You need also to check your power lines and your turn on signal as well. And also the sub to amp lines at both ends, it could be even the sub. Test everything and see what you find.

1) Yes you have to account for the displacement of 45s but in most cases this displacement is very low. 2) If you add damping to the interior of a ported box it results in a reduction of output around tuning, it also can reduce port noise in a misdesigned enclosure, it is beneficial for flatness but it will reduce output and therefore usually is not used. I will look into the modeling of the P3s however it would be necessary for me to know what amp are you using.

The best way to get flat response is what you did, run an array of high diameter subs in sealed if you don't mind the larger box. To me two standard 12s in a large vehicle cabin is insufficient but if you have to work with them then the box is the way to get them going lower by tuning low with proper port area and going with the maximum allowable internal volume. Once the box has the ability to play low you can use a DSP to try to even things out if needed, and hopefully you will like the outcome.

Depends on what 18 and the maximum dimensions of the trunk. (h,w,d) 89

You can use a 1500W RMS amp no worries, just no clipping, subsonic set to 28Hz, careful with bass boost and other tone controls after setting gains.

So here is a suggestion, power: to sub(s) rated, amp subsonic filter to 28Hz, minimum port clearance 3 1/2", figure is just an assembly guide: NOTE: If building outside you need to build a cardboard dummy box to test fit before building the real box. NOTE: Box can still be used with better subs for a future upgrade.

OP post the model for the speakers and also the model of the amp. Maybe you will need two amps but let's see.

I would definitely look into seat lift kits to fit a box of proper size for those subs, make that enclosure with 3/4" birch wood, dual layer for the baffle, proper bracing inside, specs are 4 cubic feet net 64 square inches of port area tuned to 32Hz, I believe the sub cutouts need to be 7.34". Depending on how much space you find to work with I could fine tune that spec to increase output and low end extension as possible.

So here is a suggestion, power: to subs rated, amp subsonic filter to 28Hz, minimum port clearance 3 3/4", figure is just an assembly guide:

That box is designed for SA-8s as a result the X subs are going to find themselves in a too small box with low port area and tuned too high, surely above the 36Hz specified as driver displacement for X subs is larger, maybe it will get to 38Hz or more and could experience some port noise. Used to a large box tuned very low you will definitely notice a loss in bottom octave bass. Regarding to your question, you can always strengthen the box but if it is made of cheap thin 5/8" wood then it is not impossible. Lastly the pre cut sub holes cannot fit X subs, you will have to work on the holes. Overall that box seem expensive and not adequate for the subs. I would lift up the seats and do a proper custom enclosure for those subs and even then you probably have better bottom octave bass right now than you will ever get with some 8s.

Should be fairly easy to determine what's at fault just swap parts, you have another amp, test with that, use another sub and test, check your gains, do it fast so that you can send the product back for warranty if necessary.