So, you're running EACH one on the hlg-320? Have you driven them 100%? Which heatsinks? I know they are capable of being pushed a little over 300 watts, but I thought with that standard heatsink they come with, it was 225 watts max? I know the ratings on these boards are very conservative. LED Gardner did a torture test on the qb288, which is rated at 135 Watts Max, he was pushing 600 watts into that thing and it was fine. At 670 watts it burnt out, didn't catch on fire or anything, just stopped working. So they can take four times the rated wattage, which I would take to mean that thermal runaway isn't really a concern on these boards. That's why I like to run them on constant voltage drivers in parallel, to get the max wattage out of them.