4x mtx thunder 7500 12?

I kinda get the impression you aren't sure of what to look at when you're faced with a response graph like the two I linked to.  You still seem to be inclined toward rebuilding the enclosure for more space and re-aiming the subs.  At the same time you're saying you want deeper, lower bass.  But whether you know at a glance what those graphs tell you or whether you don't have a clue, I'll go over some telling points briefly here...

Open up the https://www.the12volt.com/installbay/uploads/t7512-04at1.25x4.jpg, which shows the response of THESE subs in a box with 1.25 cu. ft per sub...a total of 5 cu ft.  Look at these 'tells' that will tell you without a doubt that you will not be enjoying deep bass from these subs no matter how big the box is:

(use the 'expand' icon in the bottom right of the screen to clarify the image)

• Parameters from the manufacturer's data:  look at the Fs, which is 45 Hz.  That is the resonant frequency of this sub and it doesn't play anything lower than that freq.
• Qts = 0.846  A high Q that tells you that this sub will work the best for its design in a sealed box.  Venting is an option, but the gain is minimal at best.  Better gain is had just with the car's natural frequency (cabin gain), which is a freq range in line with what this driver does best - SPL.
• The chart:  look at the left column which reads in decibels.  The -3 point is F3, which is considered the lowest frequency this sub can play because it is at half power at that point.  You see that it is at 45 Hz, and this is with the 1.25 cu ft per sub you're aiming for.
• The distinct hump, or peak, in this response shows the range of frequencies that this sub is designed to deliver (frequencies are shown along the bottom of the graph).  L'il John likes it a bit lower than this.  You can see that this sub delivers in the range of 50 Hz up to the crossover point.

Instead of spending more time and money reconfiguring the enclosure, you should switch to a subwoofer that will provide the bass response for the type of listening and type of music that you like.  Look for parameters with a lower Fs in the 20s or 30s and a Qts between .3 to .5.  You'll find that a lot of subs will fit this category.  Go a step further and narrow it down to subs that like a .8 cu ft sealed enclosure and also have a long throw like these do, and you can keep the upfiring box as you have it.

You know, it would be nice if we could just look at a couple of parameters in a sub's spec sheet and tell how it will perform, but that's just about impossible without putting it in a box modeling program like WinISD and looking at it.  One of the best things to do is exactly what you are doing:  talking with friends who have used some different setups and personally listening to the results.  Nothing tells more than true-life results.

On the subject, though, of Fs and how it relates to how deep the bass response can go:  you can't always tell by the Fs of the driver just by looking at the number.  I picked another sub at random just to see what it looks like modeled in the program.  The Memphis M-class MC124, a single 4 ohm sub similar to what you have.

http://www.memphiscaraudio.com/products/details.asp?cat=&id=50&p=2 and http://memphiscaraudio.com/LiveImages/114/7/DocumentURL.pdf for the parameters.

You see that this sub has a low Fs of 28 Hz...but look at the response in the link here:

memphismc124-mtx7512.jpg  Open up the response I gave you in the last post to compare.

All three of these use the same 5 cu ft box for four subs.  The colored lines you're looking at are this:  yellow is the Memphis in sealed.  You see that even though it has a much lower Fs, it still gives a response very similar to what you now have with those MTX 7500s.  F3 is at about 45 Hz.  The SQ is better, though, because there is no 'hump' in the response.

The green line is the same box size but vented, and using the Memphis sub.  There is more SPL and low end extension because of the vented box that is tuned to 30 Hz.  The difference that you see in this graph between the yellow and green lines doesn't look like very much, but it is a big difference in output if you want deep bass in the popular music we listen to now.  The F3 hits lower at 39 - 40 Hz and makes a huge improvement in those deep bass hits.  (This is my way of answering your question above about comparing this sub to your 9500's).

The third line, the blue one, is your MTX 7500 in the same 1.25 per sub box but vented.  It is tuned to 40 Hz.  You see that this extends the low bass output...the F3 is at 32 Hz.  But there's a big price to pay for this low end extension.  Look at the huge peak in response that happens when you port this particular sub:  you get a peaked response of over +5 db.  What that means is that even though this sub/box combination will perform to lower bass, the benefit is outweighed by the increase in db's at the higher sub freqs.  You would have to adjust volume because of that peaked response, which would further make that F3 point seem more like a F8 point...8 decibels lower than the majority of sound that you're getting.

Getting around this overly-peaked response is just about impossible in a vehicle because of the huge box it would take.

That all has to do with the whole combination of driver parameters and how they will interact with each other.  The biggest differences between these two subs is the total Q which is given as Qts.  The MTX's very high .84 is a sign that getting any kind of smooth response will be impossible, whereas the Memhis' .46 is a middle-of-the-road number that gives you a clue that you can work with this in either sealed or vented applications.  It doesn't boil down to something that simple...like I said it would be great if we could just look at a couple of numbers and know what to expect...but if I saw these MTX parameters and it's response I would think "SPL, how loud can we make this thing get".

In general terms it works like this:  The box tuning frequency defines the frequency below which the driver has no damping control.  But the subsonic filter, like other filters, cuts on a slope and will start the cutoff slope above the point at which you set it.  There will always be some play in where you can set that filter point.  It doesn't have to be set at the Fb and is normally set below it.  It should be set somewhere between the Fb and a point one octave down from Fb.

It's best to use test tone freqs to set this and any other filters; this filter should be set using very low sub freqs.  You can watch the cone excursion while you adjust the ss filter and see if you are setting it too low by making sure the cone doesn't move in and out wildly at any point.  You would definitely see some out-of-control movement if you have the ss filter set too low, as long as you are inputting low enough frequencies to do a proper test.