The Beyer DT770 is a ported transducer. The port is a small hole about 5 mm in diameter located directly above the yoke attachment point on the rear of the left phone and on the front of the right phone. The size and length of the port determine the resonant frequency of the air in the cavity ported by the hole. If you insert a length of hollow plastic tubing, about 12 mm in length, into the hole (heat shrink tubing works well; I flared (stretched) the outer end of the tube to keep it from accidently being pushed all the way into the air cavity.), the resonant frequency of the air cavity will be lowered. This will eliminate the one-note "whump" bass and replace it with a more realistic (tighter and deeper) bass response. I experimented with different lengths of tubing. Maybe you can find an even better length to use.

My solution was to get rid of them years ago and get better headphones. Your solution sounds interesting though.