I don't believe in knock down power. What I do believe in is tissue damage and hydrostatic shock. The latter is achieved and increased by velocity + frontal mass of the bullet.
Each of us have their own opinion on this but there is a reason you see a lot of bear guides carrying Marlin Guide Guns in .45-70 or 450 Marlin chambering rather than a 300 or 338 WM.
And also the reason I like using a .54 or lately a .58 on my black bear hunts.