A few of my impressions:
1) This was my first experience with the HD600/650. The hype is justified. While listening to the 650, I felt the infamous "Sennheiser veil" was a bit overblown. However, I immediately went back to my 400is afterwards, and the difference was clear. While it was easy to be seduced by the smooth sound of the 650s, I don't believe it would work for me as an every day headphone. Still, I fully understand why people fall in love with the sound.
2) HD800...I'll agree with Theogenes here. I was pretty blown away/impressed by what I was hearing, while simultaneously realizing they weren't something I had any desire to own or listen to on a regular basis.
3) LCD-X. My favorite headphone of the meet, hands down. Sometimes headphones just sound "right". These checked all the boxes for me. And, somewhere, somehow, my wife knows I'm typing this as she rolls her eyes and says, "Of COURSE, you fall in love with the headphone that cost $1,700".
4) HE-560. Having just bought the HE-400i, I was really dreading hearing these headphones. And, my worst fears realized...they're clearly a better headphone. But, thankfully, I don't consider them to be $400 better. *sigh of relief* Like others, I did an a/b comparison with the open and closed grill versions. I'd have to do more listening with a wider selection of tracks. The increased air in the sound with the grill mod was very welcome, but the extra bass...I'm not so sure about. It seemed a little rumbly and I really prefer a tighter bass sound. But, this could have been completely track specific. I'm sure I'll perform a grill mod on my 400is at some point which I'm guessing will have a similar effect.
5) Alpha Dogs - I liked them. A lot. If I had any use for a full-sized closed headphone, this is where I'd look.
6) Amps - I was on a mission to find the best amp pairing with the 400i. Truth is, they sound pretty good on most everything. Guess I'll stick with the Asgard a little longer until I find a combination that really works.
So, time to upgrade my source, perhaps? The HRT MSII has been around for a while. It's practically begging to be replaced.
Advice, gents?