I picked up my 6XX for $199 last week.$218 after all tax and S&H
Break in: 20 hours of burn in unified tone, soundstage, sq and frequency response. Out of the box they sounded bright, brittle, and disjointed in comparison. Update coming after 100 hours of burn in.
Sound: Warmth of tone and slight mid-bass bump works well with my preferences and vinyl source. Soundstage is good, not spectacular. The Presonus HD7 are slightly better in that aspect even though they lack in most other aspects wrt the 6XX.
Tonality, instrument body, detail, timbre, are all very good with the 6XX.
Amp/Dac: Senn HD 6XX are an excellent choice to build a solid system. Upgrades to DAC, cables, and amp, all are audible and the 6XX reveal differences among various components.
Can spend $1400 to upgrade dac/amp and then have a solid foundation with which to jump to higher level headphones that need better source components, while still being able to enjoy much of the source improvements with the 6XX.
30+ hours of burn in and the HD 6XX have settled into their sound. Slightly dark, rolled off highs, not for bassheads. Lots of detail, harmonic realism, excellent timbre.
Sound good on rock such as Time by Pink Floyd and Clash. Jazz such as Miles and Coltrane. Old fake-mono Pop via Wrecking Crew and their 1000s of hits.
But they really shine on piano and orchestral works.
Sound as though they were designed to do well on Beethoven piano concertos.
Not a surprise from Austrian or German firms.
Just a really good headphone and for under $200 nearly a "Must purchase".
If you want "audiophile" level sound this is where it begins.
If money is a concern then save up and get these first. Amp and dac can come later.
Not for portable setup - home amp only.
For portable I still recommend CAL! as decent all-arounder and cheap enough that you won't regret forgetting them at your airplane seat. Also driving the 6XX from a phone would be a mistake. Needs an amp.
Edit:
One point I fogot is that the 6XX has a non-technical aspect.
It has toe-tapability.
In that, I find my foot tapping in rhythm to the music when listening via the 6XX and not via any other can I have ever heard or own. That ranges from vintage Beryerdynamics and Koss and AKG to modern planars and the current 650 and all in my signature.
This reaction occurs across all styles of music on the 6XX.
A highly recommended headphone.