I recently got a pair of renewed HD 660S2s for $399 on Amazon and I'm really enjoying them. I'm primarily listening to newer artists including Lorde, boygenius, Julien Baker, Halsey, and then mixed in with some older artists like ABC, Florence + the Machine, R.E.M., Silversun Pickups and so far, for almost all of the tracks I've listened to (several times), the HD 660S2s do not really leave me wanting for more.
Halsey's Ya'aburnee feels like a great example of the headphones to do well in both the mid range and the bass (sub bass?) without struggling. I'm running them off of a fairly lower-end, entry-level FiiO K11 combo DAC/amp as that's all my budget really allowed for at the moment.
I'd love to hear what others are using to power their HD 660S2s and any recommendations you all might have. I won't say that these are "end game", but I do feel like these will last a very long time and are a great value (to me at least).