I read somewhere that spraying rubberized under coating (found at auto part stores) inside of a sub box help with cabinet resonance. Is this true? Are there any benefits vs using just poly fill or maybe doing both, fiber fill and using rubberized undercoating. Is it the same stuff as this at part express? Damping spray If so, the price difference is huge, you can buy rubberized undercoating for about 3-4 buck at Pep boys.