Its not the best example imo. Yes some prefer HD650, but HD800 is technically much better than HD650.
HFi780 is also better technically than HFi580.
This is not the same driver, 50 mylar vs 40 mylar Gold plated. You can prefer HFi580, but technically speaking, HFI780 is better imo. HFi780 and HFI680 are similar.
And yes, for my taste, HFi580 is much better than this overhyped M50.