2600W is way to high for air-cooled S9s. If you are looking to overclock those older S9s, I would try 1400W-1600W for the 13.5 model and 1200-1400W for the 11.5 model and see if you are happy with the results of hashrate and efficiency.

Never run more than 90% of the rated power of your PSU by the way.