The Black Max wheels are based on 6" x 6" go-kart wheels so there are quite a few tire options if we are willing to use a slightly heavier tire. A very common garden tractor tire is the Carlisle 15x6x6 which costs less than the Black Max tires but has a lot more beef. It is tubeless, and the wider cross section makes for a pretty simple install on the wide Black Max wheels.
These photos illustrate the increased diameter and tread width of the Carlisle tires. They are also designed to run at low pressure and I've found 10 psi is all that is needed for the XL. The greater volume of the tires also provide more shock absorption and have more than enough load capcity to accommodate the XL at gross weight.
Time will tell if the tread lasts longer than the Black Max tires. They certainly give the XL gear a more substantial appearance and should handle rough strips better than the smaller tires.
Update 9/24/12; The Carlisle tires are the ticket for the Black Max wheels. Very little tread wear has occurred over the course of an entire summer of flying off a grooved paved runway. The tires are much better at cushioning rough field landings than the lighter tires. Recommended.