I have been demoing the Sonarworks plug-in with Windows Vista/jRiver for my HD800 and HD650.
My HD650 is 10+ years old, but with new pads. I bought the HD800 new a year ago (it came direct from Senn -- so has a high SN; don't have it right now). I drove both headphones with a balanced tube amp, SinglePower SDS-XLR (HD800 with Cardas Clear and HD650 with Zu Mobius). Source is a TEAC UD-301. I tried different plug-in settings but ended up preferring the defaults (flat response, 100% dry/wet etc.) for both headphones.
For the HD650, the plug-in does not make a big difference to my old ears. I notice increased bass and more sparkling, but grainy, treble. But as far as overall tonality, the plug-in does not change the HD650 much. I prefer the HD650 without the plug-in. YMMV.
For the HD800, I can't help but wonder if the people selling slightly used HD800 for $700-$800 tried Sonarworks. It makes a HUGE and VERY WELCOME improvement. With Sonarworks, the HD800 has very defined/tight bass but with restrained highs. At first I thought the highs are muted but, after hours of listening, I realized that all the resolution is still there -- I just had to adjust my perception. The tonality of the HD800 with the plug-in is now much closer to the HD650 without the plug-in, but on steroids.
Prior to the plug-in, I was using the anax mod with the HD800. I of course can't directly A/B the anax mod with the plug-in, but the anax mod sounds more like the plug-in with the optimum response. Except without the bass of the plug-in. I much prefer the plug-in to the anax mod.
Highly recommended for HD800 users. For other headphones, there's always the free trial. Thanks
@RudeWolf !