I’ll put this here as well.
My attempt at incorporating the Emperor’s theme with the cue where Vader’s helmet is shown.
Here’s the combined music, using the front stereo track from the movie:
And here’s just the added music so you can mix it in yourself:
I read an odd thing regarding this in the TROS visual dictionary.
“Kylo sought to commune with the helmet to contact the spirit who once inhabited it. Though Kylo had long been plagued by voices - invasive words from beyond that gnawed at his doubts - he heard nothing from the charred helm, despite his frequent and intense meditations.”
This seems to contradict the quote “Show me again, the power of the darkness”, implying that Kylo has been shown something by the helmet or otherwise.
Minor TROS spoilers
TROS tells us that Kylo has heard Vader’s voice inside his head, and I assumed this was through the helmet, but perhaps that is not the case. In TROS Kylo touches the helmet and sees flashing visions, so maybe thats how Kylo has been “shown” things, and the voices are a separate thing. Adding the Emperor’s theme to the helmet would contradict this.