Call and talk with Mesa's tech department, nice, helpful bunch. Chances are the woofer runs full range and is not fed through a pass filter. This would mean changing the woofer would have zero effect on the crossover.
Just call me B-String 2
GK Club #488 Big Cabs #175 Peavey Amps #92 50+ Club #44 Grumpy Old Fart #2