Many here love the Ember and I'm no exception but for the HD-650 I find the BH Crack to be a better match. I think its almost like they were designed to work together almost perfectly. I also have a Lyr2 which is another good amp but for the HD-650 the Ember and BH Crack are better matches.
If you can only get one amp I would go for the Ember, the most versatile of any amp I've had the pleasure of trying, this is even after visiting my friend in Japan this pass week who has stuff that I only dream about and gave me an opportunity to try some fairly high end equipment in a purpose built room.
One thing I will say about the Bifrost and either the Valhalla2/Lyr2/Asgard2 is that the stack looks really nice.
If you are on a limited budget or just want something that is much better than out of computer headphone jack I would look at Aune T1, it was recommend to me by another here for a friend on a budget and it has worked out very well. My friend only has 2 headphones, HD-650 and K7XX and the T1 work quite well with both.