ilkka has some numbers here for the ultra: http://www.hometheatershack.com/forums/home-audio-subwoofers/6656-subwoofer-tests-fall-2007-a.html
they are 2m measurements, so add about 5db for a 1m number
this is a rough model of the tht from hornresp showing where the system is limited by the 15mm xmax of the driver (red) and 500 watts (black). it is NOT a frequency response. it is max spl in 2pi space for rough comparability to the ilkka numbers.
my interpretation is the ultra is higher at 20hz (with 20hz tuning), the tht pulls ahead by ~3db 40-50hz and by several db above that.
the lms has a high power amp and a long throw driver. the tht uses a modest xmax driver and won't require as much amp.
i don't see any reason why you couldn't use them together, so long as the delay on all the other components was about 1ms per foot of horn length on the tht. i think it is about 12 feet long, but i don't remember.