Re: Bitmain's Released Antminer S9, World's First 16nm Miner Ready to Order

Is it possible to modify the clock frequency of the S9? (I'd be interested in underclocking it)

yes you can.

Ah, silly me it's in the manual indeed:
https://www.bitmaintech.com/files/download/AntMiner%20S9%20Installation%20Guide.pdf

However they only indicate 550MHz / 600MHz / 650MHz. Are those the only options or could I enter, say, 333 MHz ?

600
593.75
587.50
581.25
575.00

all the way down to

300 all pretty stable count down by 6.25 each setting.

I have run at 400 with no issues