I managed to underclock my autotune 13TH S9 by first upgrading the firmware to the 550MHz fixed frequency firmware.
Thereafter ssh into the miner. edit /config/bmminer.conf and change
"bitmain-freq" : "475",
Maybe one can also undervolt by playing around with
"bitmain-voltage" :
Also one can change the directions of both the fans and the miner should reduce fan speed.
Do not change the directions before underclocking though, as the miner will overheat.
I tried this approach of applying the 550 fixed firmware on a Couple of autofreq 13.5 units from the March '18 batch and it worked fine, leaving them at 550.
Hash dropped a ton from 14 down to 12.
You can change then change speed via the config page, 550, 600 and 650, or config file. If you set any other value in the config file other than 550, 600 and 650 then it gets erased to "" and defaults to 600 freq, which is kind of strange for 550 fixed firmware.
Bitmain-voltage is ignored, as bmminer calculates the power based on the frequency.
Antminer-S9-all-201705031918-550M-user-Update2UBI-NF.tar.gz
I did this for a bit of a test, I don't see a significant reduction in heat \ power usage, was trying to find a sweet spot.