i was thinking going close to 3.5 to 4 cubic ft. at 40hz...maybe 32"x17"x14" which i think would be approaching 3.5ft internal volume.
at that height the drivers would probably be 5-6" higher but spaced the same distance apart.
this is assuming the omega or the JBL would fit on a 17" baffle.

do you think there would be any noticeable effects to the midrange going to a 4ft enclosure if the driver spacing is kept the same and the drivers are no more than 6" higher than your plans for the pro cabinet.