Agreed. Lower the box if you can, try to fall 'down' from the box rather than jump 'up' onto it. also, don't break at the hip - you get more glute if the plant leg is straighter and the foot stays under the the hip.
ya i was trying to keep my leg straight, but my body wouldn't let me.. thats why ive started doing single leg stiff leg ankle hops (nice and light for now) to get used to that position..
in that video, on the few jumps where my leg was straighter, I can definitely see more rebound.
peace