I just started burning in my new HD650's and while I realize they're nowhere near their full potential they already have me very impressed. After reading many reviews I wasn't expecting some big revelatory moment when I tried them for the first time. I will say, wow they're comfy, but aesthetics aside, the sound is kinda like a really beautiful woman's face. There's nothing that really stands out, and then you realize that since nothing stands out there's no major imperfections, and that's what makes these cans so perfect. The sound is just so in the middle ideal (not average) that you don't notice anything except seamless music flowing into your ears. I'm no expert reviewer or anything like that so I won't attempt to write a modest review on cans that I'm sure have been reviewed over and over again by much more expert listeners than myself.
I will say that I'll still keep my Grado SR225i's just cause they have that super punchy fun sound for certain genre's but these 650's are definitely a keeper. I don't have any money for extra amps, but for the time being my HR MicroStack seems to drive them quite nicely. The P-51 is a great portable amp for my Grado's and UE's but it doesn't do these Senn's justice.