The rolled of treble is my biggest dislike about these phones.
I would be very interested in the details of your changes if
they cure this.
The Dac I use is an outboard device[a modded comercial unit..
the midiman flying calf ,using the optical link from a sony pdcp 925.].
heres a picture.
The picture shows it attached to a volume control for a pair of
Beyer931s which I also use with my portable ,the interconnects
are Short lengths of Siltech 4/120 silver cable.
The dacs op amps can output enough to drive the beyers
directly hence the volume control.