I'd be thinking salt over PH in your situation. Any difference in Alk between the two mixes?
I had 3 torches two weeks ago myself, a single head, 2 Head and 4 Head. I've been slowly raising my DKH by .1 - .2 every couple days and recently stabilized my PH @ 8.2-8.3. I subesequently lost the single head and then 2 off the 4 Head. The other is doing great as is evehthing else in the tank.