Joe X

The problem with that is that two 3" ports have 14.1 square inches of port area, not nearly enough for two subs so you will likely get port noise when pushing more power.

Any decent 2K or so amp will do. Here is a compact size one: Sundown Audio SIA 1750 (need excellent electrical) Here is a regular sized 2K; amp d4s JP23 (needs basic electrical upgrades only) There's two decent ones but there are many others.

Your ideal box specs will come after testing in your current box, you will know if you want lower tuning and how clean they sound, usually a smaller box will produce better definition in the sound at cost of efficiency, Listen in and you will figure what is needed for your new box pretty fast.

Each coil individually should measure 1.4-1.6 ohm, then connected in parallel the measurement should be below one ohm, say ,0.7 ohm. You should not measure while connected to the amp.

Connect like this: If you connected like above and get 2 Ohm probably your sub is a D4.

Sundown X8v4 are probably the best they will take 1000W RMS each and hit the lows like no other 8" sub can.

That's an awesome box but for an SUV! If you want to help with the roof flex then subs forward / port sideways. The 8" pipe has an area of about 50 square inches so no good. I would test the Sundowns in that box just to know where you are at against the XFLs and the net volume, if they are overly peaky then smaller box but I am betting you are used to the large boxes sound.

That box is larrer than recommended for Sundown U 15s but so is for the XFLs LOL. Just swap them and see how it goes, port area is more than enough, if you find them too peaky is a matter of reducing the Net volume, there are many ways to do it like using blocks of polystyrene. When using large boxes one just need to setup the subsonic filter right and paying attention with power, you need yo keep within Xmax for the long term reliability of the subs.

You're welcome, 48 square inches of port area should be fine, the box should be 0.75 x 4 = 3 cubic feet net, A 3 common wall slot port 33.4 inches in length would tune you to about 33Hz.

You are using max net volume and will go above rated power, I'd use at least the 12 square inches of port area per driver, Haven't modeled those subs yet though.

Whatever happens with your car audio project you will end up a sketchup expert! As for the question, high tuning makes you louder but less able to play properly certain types of music like rap and such, more so with small woofers which usually struggle a bit more with the lower notes BUT if you play madonna or classic rock you are not affected by higher tuning. If you listen to all types of music you want a tuning in the lower 30s, when space is concern I always suggest 34Hz, it plays very much like 32Hz but saves a lot of box volume due to the shorter port length. You should also factor in what is the proper port area for the power you are using which seems to be above rated now that you left one sub out, Maybe 12 square inches per sub will be enough or maybe not, one needs to see with modeling software, sure enough with that kerf you will avoid most port noise but overall port efficiency gets impacted by port area. Really when you are trying to do a small box is when a design becomes challenging, One needs to pick the right compromises, sometimes it's better to lose some space, I say a wider box doesn't hurt the usability of the trunk. As always this is just food for the thought.

Sounds like an old problem..LOL BUT yes, you could probably have 3 DC audio level 5e 12s on 15 K in that space and have an edge over what you have right now. Actually you could maybe have 4 DCL5e 12s on 20K, they only use 1.5 cubic feet net each. In any case I don't know how healthy is for your 15s is staying boxed for years and years, might want to ask Fi about that.

5" speakers usually lack lower mid bass so that's normal, you should use speakers as large as you can mount, usually that is 6.5", to find what fits your car, visit the following link: https://www.crutchfield.com/car/carselector.aspx?lp=%2fcar%2foutfitmycar%2fmycar.aspx

start with these: Set your bass boost to the zero position Set your high pass filter to the tuning of the box Go through a gain setting procedure make sure the box has no leaks and that the sub is well seated. Even at 4 ohm that amp can put out almost 5K, the sub is only 2000W so plenty of chances of over powering. Lastly post your box design with dimensions of all parts internal and external, i will check the design for you to see if something is wrong.

Really you don't want a ratio higher than 6:1 (say 6 inches wide and 1 inch tall or 6 inches tall and 1 wide for example) going from 20:1 to a 10:1 is not going to help a thing and also using two (or more) ports by itself is less efficient than using one so that divider idea would just make things worse. I am probably a bit annoying now but it's better to know this things, rebuilding a failed enclosure is very frustrating not to mention the ruined wood and effort.