Not all HD650s sound the same. I think a few others here can corroborate.
The HD650 (#2) with the better FR (without such strong a dip) had a good amount of bass distortion:http://www.changstar.com/index.php/topic,469.msg8390.html#msg8390Bass distortion is what causes slow muddy bass. I found this HD650 to have very slow and mushy bass.Not all HD650s sound the same. I think a few others here can corroborate.The graphs here are compensated to my and LFF's ears using neutral references, with input from Anax. My personal reference is a near field desktop monitor actively crossed over and EQ'd with a neutral response. There a post someone with the FR and picture of them either on this site or HF. Don't remember the link.