splotten, you cheated. :-)
the 'cube' has an 18 inch driver with 22 mm excursion and runs on 1100 watts. without increasing port velocity (i.e. keeping the port cross sectional area to 2" x 22.5" minimum), how much lower of a tuning/smaller of a cab do you think is reasonable?
taking down the size of the enclosure by 0.75 cubic feet requires an increase of port length inside to hold tuning and that requires something like 0.5 cubic feet, so the net decrease in cab volume is only about 0.25 cu. ft. but the sensitivity/performance loss is as though 0.75 were taken out of the cab. in my estimation, it is right about at the minimum as it is, without going to passive radiators of course.