Yeah GM, about 11ft seems right to me.

I had done a quick sim on the 515B in an 11ft vs 8ft box and it looked pretty good. Group delay was better in the smaller box, if that matters. Tuning in the mid 30s for both boxes. Didn't look at damping.

So Mr. Monster you should be able to go with a smaller box and still sound nice. In a simple box you lose the A7 style bass flare which does take up some room. Would my 40x22x19 box fit your needs?

If you are running your cabs with wide open vents now, you really aren't getting much bass. What you may be getting is a nice hump circa 60Hz. A better tuned reflex box will give you lower, cleaner bass. I know, been there, done that! =)