Answer:

If you want to train for boxing with resistance, use the bands instead of dumbbells.

The problem is not with the resistance but with deceleration (slowing down) of the weight.

When you box, you don't want to slow down your fist. If you hold a dumbbell, you will have to slow it down otherwise you will throw your shoulder out of the socket.

Resistance bands decelerate your hands for you because the resistance increases as you stretch them out. They actually work quite well for boxing training and I would highly recommend them for boxing training.

This is my recommended source for training bands. (Note: Use Coupon Code "rbtfitstep" to get 10% off your purchase of a band package)

If you're interested in some excellent core training for boxing, I would recommend the One-Arm Feet-Anchored Dumbbell Bench Press.