Yeah since I can't move the box because it's so big I'm guessing sealing it off is going to be the thing that has the most dramatic effect on my response curve.
I only listen to rebassed music in my car. I go to my computer and put on my Sennheisers when I get the SQ itch.
To be honest I'm pretty content picking and choosing songs within my ~10hz window and enjoying the output. With a box this large I agree I'll probably hit mechanical limitations first. In a few months I might play around with the idea of lowering the internal volume and raising the tuning a little.
I really want to just see it on full power and sealed, and see what that does. I think it's positioning and not being sealed off could be messing with it some. What I would do is make the box as loud as it can be as it is now, by doing all of the things to make it proper, and then maybe modify the box. I'm a little suprised it wall fall off that early, from every time I've done this type of set up. The box is tuned low, but it's not tuned that low. I suspect some kind of cancellation issue here. It'll be interesting to see. It's so hard to predict every factor. I wonder what a burp at 30 hz or so would do lol. Make your car look like jello.