I had the chance to compare the Sennheiser HD650, 800 and 800s, and all 3 were connect to a 3000$ sennheiser amp, hdvd-800 I think. Anyway, the 650 were not close to the 800 line... The clarity/detail/musicality was just not there, the 650s sounded so muddy and bloated in comparison... Absolute let down. Now, it's just a pure impression as I didn't have the 400i to compare, and was listening to their music, but although some have said that the 400i is comparable to the 650 in terms of clarity/detail, I find that hard to believe...
As I mentioned though, the amp powered 3 headphones at once, so maybe the 650s weren't driven properly or something, but my quick impression would be that the 400i sound quality is a bit closer to the 800 line than the 650... Like if the 800 line was the reference at 10/10, and the 650 was a 6/10, the 400i would likely be like a 7.5 or 8/10. I'd say maybe the 650 would be more comparable to AKG's Q701, the AKG is no slouch, but vs the 400i it's just not as transparent/fast/detailed... Bit more 'lowfi', which is more how the 650 sounded to me...
Also heard the Beyerdynamic T1p, and it was driven by a small portable fiio or something, it also failed to impress, I'd say the amp was likely the culprit as their pricepoint is quite higher than the 400i, but yeah, it was ok/nice, just not impressive when thinking of it vs 400i. The Senn 800 line was amazing though...
Also heard the Hifiman 560, driven by some ridiculous Chord amp, with the silly name of 'Dave' it looks like (not kidding!!!
http://www.whathifi.com/chord/dave/review) and wow now
THAT was something. Smooth sound, detailed, nothing bad stood out, very pleasant sound! But then, I'm seeing 899$US for 560 where I got the 400i for 240$US... Definitely a sweet setup though, hard to tell which of headphone or amp added some
magic, but my guess is that both were doing a great job. Definitely a good step up vs 400i, but then who knows how much better the 400i would also have sounded on that crazy dac/amp...!
The Sennheiser Orpheus HE-1 was also fun to listen to. I almost bought it, but then realized it would not fit on my desk, also they only had it in white soo, I had to leave it there...