The hd650 is one of the gaps in my HP journey. I probably should have got it earlier on but waited and was intrigued with the hd598 and tried it instead. Didn't like it much.
Maybe I'll snap a used set of 650's sometime. Got to get a proper desktop amp first though. I've been going round and round in circles trying to get what I think is best. Just haven't pulled the trigger on anything still.