"If someone was going to build some mid-bass cabinets with the JBL2226, what would be a good enclosure size and what would be a good way to power and implement them with an existing monitor setup?"
unless you have a heck of a monitor marty, it isn't likely to keep up.
a 4 cubic footer tuned to 50hz will provide a -3db point at 50hz and about 97db 1w1m 2pi space above that point as high as you wish to run it. in room, probably more, depending on your room and placement. in such a cabinet, xmax is just barely reached at full rms power of 600 watts.
have you grabbed winisd yet marty? that would help address many of your questions with respect to how different driver/cab combo's perform.