Dampening the enclosure
The DT231 is said to sound good considered it's price. I was fairly sattisfied with it, but when I opened it up and found no dampening, I had to see if some enclosure coloration could be removed.

The earpiece is easily removed by pulling it off. Beneath we find 3 screws to remove, and the undampened enclosure is revealed. I inserted a piece of fairly dense foam measuring 4x4x1cm (that would be just above 1.5inches and a height of about 0.5inch). I assured that the internal wires were safely positioned, and assured that the cable hold was properly mounted on the left can.

I have not experimented with different types of dampening. I chose dense foam to avoid that the drivers would be lead to think the internal volume was increased. This probably will happen with a tiny ammount of loose wool.

The result was pleasing. Harmonic distortion and enclosure coloration was reduced. The bass became thighter and the mids cleaned up a good bit. I now found the headphone more impactful and balanced.

Grille mod
After experimenting with different pads on my Alessandro MS-1's (Grado) and listening to the diffrences in sound, I just couldn't help cutting a bit in my DT231's. I expected more direct bass and more emphasised higs. I got som scissors and cut out four intact squares from the grille. Be careful during this operation as you have a transparent diaphragm just beneath the grille.

The result of this mod seems to be as I anticipated. The bass became a tad more boomy, but not bothersome, as it was thigthened up a bit by them dampening mod. The higs were really released, the phone became a bit more dynamic, which is to my liking. Hi-hat/cymbal suddenly became more realistic, this has so far been my main objection to this headphone. But this has introduced a slight case of listening fatigue. If I were to try this mod again I'd try drilling up the size of the holes in the grille, rather than cutting out a piece.