ASC Listening to masked tapes at 40% volume

[InTheZone]

Listening to the masked tapes via my Sony wh-1000xm4s at just 50% volume gives me mild ear pain the next morning, and I hear sounds like I've got tinnitus. 

Is it okay if I listen at 40% with noise cancellation on?. I am able to still hear the lows of the ocean surf tape.

You should be careful with noise cancellation. It sounds quiet but it is actually generating sound to cancel ambient noise. I've heard of people getting tinnitus just from wearing noise-canceling headphones all day even with nothing playing through them. I would not use noise cancellation all night. See if that fixes the issue.