I figured as much. Thanks.
I have just modelled that 18" Ultimax driver in your 3.6 cu.ft box and the 6.5 cu.ft. optimal box. The models are attached. The top file is your box and the lower one the optimal box.
This driver is for sealed application.
The 3.6 cu ft box has significant compromise.
The Qt of that driver is on the high side at 0.7 and in your box total Q is 0.785. In the optimal box of 6.5 cu.ft. it is 0.707. That may not sound like much but when you get a Qts of around 8 or higher the bass in not tight.
The bigger issue is that in the smaller 3.6 cu.ft. box the pressure gets so high that cone excursion flattens out, and does not increase. So that means you can apply more power, but the sound output does not increase. This could result in driver failure if you pushed it into this range.
As with all sealed boxes the F3 is high and so a 12 db per octave boost has to be applied below 35 Hz, and then high passed at 18 to 20 Hz. So you need a plate amp with boost.
This 1000 watt plate amp would do the job.
The problem as I see it, is that there is significant cost to this project and if you use your existing box, the final result is sub optimal, although not dreadful. However the funds might be better put on a new sub. That is for you to decide.
