Your stock speed is 13.5TH/s. There are 14TH/s auto-tune machines out there, after all.

What S9 batch did you get? You said 631MHz, so auto-tune, but at what stock speed? You may just have better hardware than I do. I can only get 12.2TH/s @ 600MHz, 840mV.

I'm on North American electrical, so staying under 1440W total power is critical to me.

Bitmain's GitHub provides enough code to build your own bmminer, and that means enough code to hack your own bmminer.

Voltage can go as low as 7.94V or as high as 9.43V. So yes, you can undervolt, and it remains stable but not full speed.

Data points: 13.5TH S9 @ 575MHz fixed frequency and 8.4V gives me a consistent 12.05TH/s. Same hardware @ 600MHz and 8.55V gives me 12.7TH/s. These are both significant undervolts; a stock 13.5TH/s S9 at fixed frequency will take 9.00V.