I have 650s. Used to listen on Naim Headline powered from 552; it was fine. Sold that, moved to venerable Cyrus One as a stopgap and got a totally different presentation; very good nonetheless. With my newish Myryad Z40 I'm listening to different cans; a world away from what I have been hearing these last 6 years. Sources are Lector valved CDP, Naim 01 tuner and N.A. Dais/12" PU7 with Transfig. Proteus. Oh yes, a lot of tv too.

Conclusion? Headphone amp's make a hell of a difference. I've tried them on my Pioneer amp. in the office and Sony in the kitchen. Functional but that's it. A dedicated h/phone amp to suit your cans is a no-brainer.