Bitmain has disabled ssh from S15 and now also uses older miner signed firmware files. so users can only use the original non-overclockable version. that's why everyone wants to get in there 
That's the fact they don't want their miners to use their firmware to access SSH and overclocked the miner's speed and increase the efficiency.
So that the overclocking method like 18th/s in s9s will never work anymore in the latest version(Unless if someone will show/reveal the hidden menu from webGUI to enable SSH)
S9: Also, no downgrade to an old firmware version is more possible if the user upgraded to the latest
of course with sd carte / re flashing goes - if sd slot available
of course with sd carte / re flashing goes - if sd slot available
I never heard someone downgrading s9 with low version but they released a method to make a recovery program through SD card but the firmware will become t9+ based on their guide if you use this firmware I'm sure your hashrate down to 10th/s.