I'd just call them a good counter to the brighter transducers of this world. I always used the HD600's back in the day with some very powerful amps and I had no complaints. Never did care for the HD650 though...
One odd thing though, this hybrid I have now sounds much more like the HD600 than the 650 despite having brand spakin' new 650 drivers. The baffle is an old 580 unit so that could account for the change in tone.