Would putting sound deadening material on the inside of a sub box help with anything or would it just be a waste of time? Vibration is wasted energy right? So if theres no vibration with the sub box then I would think it would be worth putting in sound deadening mat or spray.

 

I'd say it's worth it. I've used sound deadening and poly fil/insulation to allow for small volume AND thinner material to save weight. I've always gotten good results by incorporating both a spray in deadening agent and using poly fil. My 2 Type R 10s back in the day hit 138db using 1/2" mdf and using the above products.

 

purpose-made sub box stuffing is not the same stuff you use to deaden the sounds from outside trying to creep through your car's body panels. If your sub box is vibrating from the air pressure inside, brace it or rebuild it. If the box modeling software says to use stuffing, use stuffing.