I see your signature, you obviously sold off al lot of your stuff. And the only one left is the HD650! What happened?
Working 6 days a week since almost a year reduced my free time a lot. Free time if spent on music/games/movies is split between the re-blossomed loudspeaker setup, HD-800, TH-900 and so on. Impossible. Sometimes I wouldn't use anything for weeks so I figured it's better to concentrate on one headphone and sell off stuff. Timing was right as well as improved versions are on the way and prices fall. A pity as I really loved the possibilities from both headphones. Got pretty good money for the gear though.
However don't worry. Got the Gungnir Multibit incoming.
About the HD 650: I think it's the best price/performance headphone in the world. There are a few sacrifices to be made however when coming from the HD800s. Most noticable is the bass section as in refinement, extension, "detail" (let's see what Dynamat changes), then soundstage both in imaging and most noticable: depth. I miss "frontal projection" with SBX when watching movies or trying out a game. After years with the HD 800 such differences aren't easy to "stomach", however it's easiest with music. That's where the HD 650 ultimately proves to be THE headphone to have out of and before all others. I cannot emphasize how clean (except the bass distortion) this headphone is and how the tonal balance is outright ideal (again, with the exception of the bass). More than a decade old but showing more detail than a few kilobuck headphones, disappearing and just letting music happen.
Seriously, get a Vali/Val 2, Bottlehead and a decent DAC starting from the Modi 2 and you'll be away from the competitors in its price range. Like far away.
back to the topic: The HD800S sounds like the perfect headphone to me, however I refuse to buy it above HD 800 price level. Sure Sennheiser is not pulling an Audeze or Hifiman but forced bundling of a SECOND cable might be clever marketing (higher perceived value) but it's obvious to most of us how lame this is.
I also believe that the introduction of the HD800S will pave the way for a new flagship in the next 2 years. So better wait and enjoy the HD 650.