Well, I just bought a pair of these used and didn't get a very good deal as they're only 300 shipped right now...but the advantage I do have? I'm listening to them right now!
I haven't been blown away by headphone equipment since my upgrade from senn hd280s to beyer dt770's. These are monster cans. I've had 650s for almost a year now and I have no desire to keep them around anymore. If you're torn between these and 650's you're getting a great set of cans either way but the 400's are on another level IMO. If 650s run about $450-500 then these should be $600-750.
Your opinion may vary so definitely try if you can. Anyway, happy thanksgiving from a VERY happy new HE-400 owner. I can finally just enjoy music instead of wondering how I could improve the experience.