You're right, I should have been more precise. I meant clipping above 1W though there is sufficient drive voltage.I think you need to specify 'power tube clipping'. Otherwise, we see clipping at the 1W point yet it could be clipped by a preamp tube. In that case it's not really power limited. This is where the measurement you suggested at the power tube grid is important.
If we have preamp clipping, this would be my "case B", namely drive voltage limiting.