After several says trying different firmwares I can confirm the following;
With original bitmain LPM firmware + original bmminer, just changing the frequency inside advance panel and modifying through css the fan speed control to 100%, right now I have my two antminers s9, one 13.0TH s9 running to 675 = 14.237TH and 73C and the other s9 14.00TH running 750 = 15.931TH and 80C and almost no hw errors and 1600w psu. So I´m not experiencing anymore the heat problems.
strange...
I have about 15 with 650FF firmware with the LPM fw on top, and highest i can clock them is about 668Mhz. Any higher and hashrate starts to drop.
Also i have 4 that are situated in a colder area, and they are so picky they wont even take the LPM mod; too cold, hashrate drops and they run stuck. Reboot and they go for some hours again. New flash with 650 (or autotune only) and they're fine...
So, temp. has a big influence also... Chip temp of 40 degree centigrade they don't like at all...