Asgard + H650?
Warm? yes.
Muddy? no
And as far as Asgard not being good for 300 ohm headphones? buahahaha. Sorry, but know what you are talking about before you talk. I generally try to keep it civil on these forums, but that type of misinformation dissemination=bad. Pairing and powering are TWO disparate things. Just because you don't think they pair well doesn't mean it's not powering well. Schitt does recommend the Valhalla with the HD650, but NOT because the Asgard can't power the HD650s. It is because Schiit had the HD6XXs in mind when they designed the Valhalla. And this is probably why many think the Valhalla sounds cold for a tube amp. Personally, I love the HD650 sound signature and don't want to counter some of its qualities. So I prefer the Asgard with the HD650.
Now as someone who owns a 250ohm and 300ohm pair of headphones and owns the Asgard, I can confirm the Asgard powers both no sweat. I listen at about 11 o'clock for normal listening. And I feel the Asgard pairs beautifully with the HD650. The Asgard enhances everything I love about the HD650. The highs still roll-off, but all the detail is all there. When I want forward highs, I'll listen to my Beyers.