The board is meant to be powered by a 14-39v PSU. The fan is unacceptably noisy at 14v but at 36v, it went really fast for a second, made a pop then stopped working along with a nasty smell.
Turns out there is no regulator or speed control for the 12v fan, it's connected directly to the supply. Why do they think a 12v fan is suitable for a possible 39v supply? Beats me.
Anyway I purchased my own temperature control board and a new fan because I love this amp otherwise.