I am also watching 38 and at around 19 minutes Kenpachi explains that
"The reason I don't seal my soul slayer is because my spirit force is so huge I couldn't seal it, even if I held it in as hard as I could."
Sounds like it is at the next level to me.
A lot of the stuff we are argueing can go either way it seems. They seem to imply things both ways in a lot of situations to keep you guessing.