Fixed freq FW adjust voltage depending on frequency. Higher freq means high quality of chips, so FW will set 8.8V per chain. If you use 600MHz, it will set 9.3V for the chain, because it expect lower quality of chips. I compiled binaries which do not respect quality of chips and up to 675MHz will set declared voltage (eg 8.4V). 675MHz and more will force voltage to be at 9.4V for easy OC without copying new binary.
So, for underclocking, copy bmminer with 840mV (8.4V) to your /usr/bin directory. It will force 840mV for all frequencies up to 675MHz.
Ahh okay so your just hard setting voltage then pick and choose which frequency is running best and you have it set by default to 675. So we could just go in there and set it to 700 etc.
Do you know which voltage would be the highest the bitmain 1600w psu can use. I want to run my s9 at 1550-1600w to draw around the max the psu can handle and hash as fast as possible with that psu