Well, I own both the HD650s and the D5000, and I just modded my Denons yesterday via the Markl Mod method. I haven't done any extensive comparisons yet, but my initial impression is the modded Denons beat the HD 650s in every way. Before the mod, I still preferred the Denons for their more engaging and energetic sound. The Senns just seemed anemic compared to them. However the Senns still seemed to have an open/spacious/effortless quality that I still really missed in the Denons. After the mod the Denons sound much clearer, much more open. Better detail, blacker background, much better controlled bass, much more balance sound. Except for the rather brittle and slightly sibilant highs, they're almost perfect.
Forgot to mention, my HD 650s have the Cardas cable.