I may have solved the problem. After mentioning I sometimes cheat to about 2 O'clock when hitting my snare head it occurred to me that the muffler happens to be located in that position. I tool a look at the broken heads and sure enough they both seem to have broken at that spot.Looking at the Royal Ace muffler, it is a small twin job and one of the felts is worn off, exposing the metal beneath. I think what may have happened is that, although I didn't have the muffle engaged, it was very close to the head and the felt-less muffler tore the head when I hit just above it.I made sure the muffler was fully disengaged today and played without incident for several hours.