I read somewhere that the HD681 and HD681 EVO differ only by cosmetic design. Perhaps all the added material makes them significantly heavier. The HD681 are really light to me, but I'm coming from some T50rp modded with a lb of clay and foam stuffed inside.
It looks like their headband is very simplistic. I ripped apart several headphones to make my T50rp as light as possible. My headband is made of a strip of light foam and two bare thin metal wires for clamping force. If you're the modding type, carefully pull the headphones apart and see what material you can toss out to decrease weight!