I don't consider the HD650 a reference in many departments. In the sub-bass area it's probably not reference material. I also slightly prefer the HD600 to the HD650. The upper mids and the treble are not without some bumps. However, I like the tone better than the LCD-2, HD700 and HD800. Furthermore I don't think the HD700 had a reference level upper mid and treble response at all. Relative to an HD6x0 model, the HD700 seems way more colored and bright. The HD800 OTOH needs some modding to tame it's analytic tone (somewhat lower treble emphasis and a little tilted response). The fact that the HD6x0 (& HD580) models have been able to hold their own even today is remarkable to me.