I'm looking for a moderately to normally warm pair of open backed headphones. I was under the impression that the HiFi HE series are in that category. It looks like the HE-400 cans fit the price range as well.
I want a warm bass response without compromising the quality of mids or highs. You can read that as I don't want the mids or highs to be more any more diminished than my K550s.
It would be nice if the headphones could be driven well by a small amp or no amp at all. But that's is not an absolute requirement.