I don't find the 600's or 650's difficult to drive. I plug them straight into everything.
Same with the 700's I just picked up used, which I'm not as familiar with yet, but the clarity increase is substantial. They may be a bit lighter in the bass, but that could be a subjective influence of the increased clarity. I haven't plugged them into the Mytek DAC yet, just direct out of my recorders and computers, so maybe these 700's need a bit more grunt to fill the bottom.
What are your thoughts on driving the 700's, rigpimp?