Had a nice visit to the Sennheiser store in SF last week, where I was the only customer. Really only spent about 5-10 minutes each with the 800, 800s, and 820 using mostly my Mojo but also their 820 amp. I'm a newbie so can only describe in relatively vague terms, but I know what I like. My HD700s are my favorite headphones I own (over Massdrop HE-4XX, 6XX, and Elex) so I was very interested in listening to the top of the Sennheiser line. Am very interested in a similar sound to the 700s and love a big soundstage, but want closed back to use on planes and not annoy the spouse who can hear my headphones in another room while he works.
The 800 seemed most similar to the 700s (though didn't have them with me to A/B and the store doesn't sell them or the 600 & 650s anymore so couldn't even try those). 800s had more bass and toned down treble and were much more pleasant than the 800. The 820 I enjoyed as much, a bit more bass, and smaller soundstage but much larger than the Elex, which is open. All 3 were huge and didn't seal well on my smallish head. Approximated what it would sound like to others with them playing while wrapped around my leg. Basically couldn't hear anything coming from them so I'd feel comfortable using them on a plane. Am now contemplating selling off the 3 Massdrops and getting the 820s when I can find them under $2000. Also tried out a Clear, LCD-X, and Ananda elsewhere and was not similarly tempted, though enjoyed them all. Ideally I'd get the 800S and 820.