Well, currently I am using the Asus Xonar essence one Muses edition, and while it sounds clear and crisp, it really did not marry well with the Denons..
I have removed the cups to reduce the resonance that seems to be around 8kHz and to increase the bass slightly, so I can listen without destroying my ears... The backside is that the sound lacks direction (its been discussed in a few threads on this forum I believe).
All in all I would not recommend the Asus Muses edition for the D7k.
(Also, the first unit did not have the low/high impedance jumper, but this was quickly take care of by an Asus correspondent on this forum without this jumper set to low its unbearable.)
So far the best audio experience with these cans for me has been with the PCI soundcard Creative X-FI Extreme Music. I know it is very...."non-audiofilic" to mess around with the equalizer, but I could really configure it so that I and all my friends who tried it had a hard time stop listening to them.
The hunt continues, and though many suggest solid state amps for D7ks, I am really interested in the Woo Audio Firefly, it looks really fancy.
@ DoctaCosmos: there is a HUGE difference between the D7k and the D2k, I too became so disappointed in the audio produced by them when I first tried them on that I felt that it probably was useless to continue the search for a good pair of headphones (The D2k were the first ones I tried in the more expensive category).
@Kikivan: The Asus Muses Edition is a dac with a built in headphone amp, however, no love to be found here.
@Oregonian: Do you have any specific models to recommend?