OK flasken, especially for you...
I find the soundstage of the HD600s and HFI-650 very similar. Depth I would say is the same, width - well, maybe, just maybe, the 650s are a teeny tad wider, and also just maybe another teeny tad more positionally defined.
And to continue in a different vein - JEEZ man, is this what you call visceral bass??? Instead of a vague 'woof' (the 600) I'm now hearing, and feeling, real low frequencies (the 650) - from bass guitars, double basses and bass drums - man I FEEL the pedal thumping the skin. WOW! They're really there now, no... they're HERE!!
OK, I'm sounding too gushing now, but honestly, I swap over to the HD600s, note the difference, and can't get them off quick enough. Oh, but the comfort of the 600s. I've just developed a seriously bad toothache, and that together with the pressure on the top of my head from the 650s headband is making for a rather unpleasant experience -oh, but the music, the music
One thing to bare in mind though, is that I'm using a SlimX and super-mini amp. Perhaps the 650s are so much easier to drive - perhaps the 600s with top end sources is really better. Who knows.