I'm using D5k's together with Asgard and the cans sound more alive, compared to the amp that I was using before - ProJect Head Box II. The current setup still includes Pro-Ject USB Box DAC.
Cant compare to other AMPs that you've listed, but if to compare with Pro-ject, the Asgard revealed many details in the recordings that I have not heard before. Also partially fixed the sibilance issue! Highs no longer get out of control. It still rolls off at times, but now I have a feeling that some tracks have been poorly ripped.
What I'm saying is that you should not rule out Asgard because of the impendance. I'm quite pleased the way Asgard sounds through D5k's. At the end of everything - it's still an inexpensive AMP. Although I have also heard that Denons and Opera play really, really well together. Too bad I dont have a possibility to audition the Opera!
good luck in your search. Let us know what you decide and how it'll play together!