Stuck and in need of assistance.

[all4spl]

So I am stuck, cannot decide what to do or which direction to even go at this point, need some assistance and I will try to make this short as to avoid the TLDR syndrome.

To start off here is what I have currently in the vehicle (a 2009 Honda Fit), (2) Sundown SA 12’s wired to 1ohm on a SAZ1500D. They currently sit in a ported box that is 20”Hx31”Wx16”D, with (3) 4” aero ports 12” long, yielding a box of 4cuft net, tuned at 38Hz subs up port back. While this has been the loudest box I have built, it’s not the only one. I built another 17”x39.5”x16” with (1) 6” aero port tuned at 34Hz sub/port up, and finally another with the same dimensions however this last one was a slot port with 49.5sqin of port area subs up port back. The vehicle is a 2009 Honda Fit, this is a hatchback for those who do not know. The car has the big 3 done under the hood with 0 gauge, as well there is a Shuriken BT35 under the hood. 0 gauge runs to the back distributed to the two amps.

I have some money saved from my recent birthday and was going to buy new recones for the subs to put them at dual 2’s and the SPL coils. As well I was going to buy a HO alternator for the car from Tenney Alternators. However before going to those expenses I have been thinking I may need to change sub sizes down to a pair of tens to really get the output/sound I want. I would like the car to play lower (35Hz) very strongly, and I would also like to car to hit a decent 145 on a termlab, which I don’t think is too much to ask out of the equipment I have or I am just crazy and an old fart.

The MAXIMUM box size to leave 4 inches behind the box (leaving a space between the box and the hatch door) is 20”Hx39.5”Wx16”D. Giving me 5.9cuft before any displacement, and from the boxes I have done subs up port back is the loudest in this car. As I stated I am not against changing subs down to a pair of 10s, I just want the two things I listed solid play down to 35Hz and a fair spl of 145. Of course anything higher in SPL would be great but I think keeping my expectation lower is better at this point.

Right now I am looking at PSI Platform 1 10s with upgraded voice coils and stiffer spiders, but I know there are a TON literally of different subs out there so brand isn’t as important to me as performance and durability. Sundown has done well for me but just not sure if a pair of SA10’s is going to give me what I am looking for.

Any input is appreciated, if questions arise simply ask and I will answer them, and thanks for helping an old man out