Well, the HD800 has a tilt towards treble, which is polar opporsite of the HD650's balance, so of course, next to the HD800, the 650 will be dark/slow. I don't like comparing headphones with an obvious difference in balance, which will make one sound bad depending on your personal preference. I for one, favor the 650's warm tilt (among others like the LCD2), over something on the bright side of neutral like the HD800. I'd choose those two any day of the week.
Have you seen measurements of both HD650 and HD800? FR aside, HD800 measures much better in all possible aspects (innerfidelity)...
HD800's technical superiosity is definitely not only about having more db in treble... HD650 are slow/veiled even compared to other darker to neutral headphones (LCD-2/LCD-3 are a perfect example of how to do darker headphones right).
You know, even Tyll prefers HD600 and sees HD650 as slightly veiled and dark... And that means something (considering his preference for warm-sounding headphones).
To sum up, all I wanted to say is that HD650 simply are veiled and slow... It's not an overstatement. IMHO.
Edited by Ron12 - 6/12/14 at 2:55am