So it is possible with other versions of S9 . Did you ever tried to limit the power ?
Everyone using Braiins OS+ is limiting the power. There is no speed control, only power limit control. You tell the miner how many watts to use, and the autotuner does the rest.
It works perfectly fine with supported miners. The S9SE is not supported as you have already been told, it is a different machine that has nothing to do with the S9, its closer to the S11 which is also unpopular.
Also, Braiins OS+ is not modded firmware, its built from scratch, so there is no way to compare it. Think wiping Windows and installing Linux in your PC.