I think the 600s are boring as hell. They are tonally accurate, and the mids are lush. Good bass, tame treble presence. Soundstage isn't all that great, though they image very accurately. They are impressively balanced, but they don't jump out or impress. I am a soundstage junkie, and I have sort of given up on analyzing cans and am trying to enjoy music first and foremost. I find that tonal accuracy, while the most important criterion for audiophiles, is secondary to other factors as far as sheer enjoyment goes, unless the unbalance is really off putting. Finding slight pronunciations or recessions in a frequency spectrum is good for geeking out, but listening to music that way is not really focusing on the dynamics of the music, but of the sound as such. I am no longer interested in doing that.
The 650s are thicker, even lusher. I find them way more unique and interesting than the 600s, because of those superbly lush mids and punchy bass. They are quite dark, which is not my thing, but they do that better than almost anyone. And I own the 600s!
As for the Fidelios, I own a pair of X1s. I like them quite a bit, and exemplify what I was saying earlier about tonal accuracy. A lot of people complain about its treble being slightly off, but seriously you have to be extremely nitpicky to mind that. They have way larger soundstage than the Senns, are better built, and have a stronger bass presence. The x2 are similar, with a bit less bass, but more controlled one, and tamer highs. I am gravitating towards brighter cans lately, though. I really like the Beyer dt880s in that price range.
I think the Fidelios x2 are still the best bang for the buck in the bracket, considering their very negligible amping requirements. If you have a good amp, then the Beyers or Senns scale better.
I stay clear of Hifiman products now, since they are generally annoyingly unbalanced, and the build quality is genuinely horrendous. They were overhyped beyond belief for a while. I own the re-400s iems and that's the last time those guys will get my money. They are terrible.