I have a HD565 and want to upgrade also.
I have it for a long time now. Some years ago, headphone for me was not that important as nowdays. I listen to music more ans more with headphone. Years ago I planned to upgrade to HD600 and had a friends HD600 for a few days. From the point of view what I do not like of the HD565 (weak bass) it was a big upgrade, maybe even too much. In the other hand, the midrange, soundstage, space ... was not that good in my opinion. The sound was probably much better for pop music, but I rearely listen to pop. I have ended up staying with the 565. Now I picked it up again. In the meantime I have changed my speakers to other boxes which has very deep and precise bass. I listen to Jean Guillou / Pictures at an exhibition on the speakers than on the headphone and I realize how much I miss at the low end. If I raise the volume, it comes up than, but than it is too loud. For sure it is not because of the source or the amp. I want a headphone with better bass reproduction.
Now I am looking at Grado, a 225i or a PS500, but in Hungary it is very rare, Sennheiser dominates (and has good price). Before I hated the on ear headphones, but I have recently tried some very comfortable ones.
Dont know which way to go. It would be nice to lease a HD650, a better Grado for a day or two at least, but I see no possibility for that now :-(