Well, I'm just a newbie in this area even though I'm reading and listening a lot right now.
I received my HD6XX today and they sound good together with the Zen Dac. The Zen Dac is able to drive the HD6XX/HD650, perhaps they would sound even better with more power but this is good for me, I'm not an audiophile
yet anyway, my ears likes this.
Usually now depending on music I listen between 10:00 - 15:00 a clock with power match button engaged (and I listen too loud, I'm fully aware of that, highest for classical music though).
HD6XX - Zen Dac combo really good for classical music (!), jazz, acoustic, rap, r&b and various soft/clean music, Pink Floyd great as well. Rock, metal, stoner and death metal was good too but definately not as impressive together with the HD6XX as for soft/cleaner music. "Problem" is my closed Sennheiser Momentum 3 BT actually sounding really good for rock, metal, stoner, death metal when running them turned off/passively together with the Zen Dac. I still believe HD6XX is
sligthly better for this type of music too though (slightly more details) but a surprisingly tight race for this type of music (or the M3s are surprisingly versatile). The bass button on the Zen Dac is
great for some songs but not for all, I try to get used to not utilizing the bass button.
HD6XX may not be the most detailed or punchy headphone out there, perhaps
slightly dark voiced (or rounded), the highs are smooth? In the same way it's too harsh/strong calling them "dark", probably depending a lot on the amp/dac too but
they don't give me any (or very little) ear fatigue which is really good and are enough comfortable to wear.
Another factor probably affecting my impression could be the fact that I find way more high quality MQA/master tracks and good recordings in Tidal for "soft music" comparing to metal, stoner and death metal which is a bit annoying. Perhaps some metal bands I listen too have a low budget.
(I listen a lot to Stoner rock too with lots of fuzz, small bands.) More famous metal bands seem to have higher quality recordings and these headphones play what they get (good or bad) without filters or dsp (brutally honest). Edit: Pantera 2003 remaster sounding killer with these headphones and the Zen Dac.
I think I will avoid going crazy right now and try more headphones (or dac/amps), HD600, Grado, Shure, Audeze, Hifiman or it could become expensive in the long run, I will stick with my HD6XX. Finally it's possible to EQ the sound, the parametric eq in Roon is working really good (trying Roon but will probably not purchase even though it's great).
Since we have a balanced input the Zen Dac I'm curious trying a balanced cable though. Problem is that 4.4 mm balanced for Sennheiser are difficult to find in stores but I will go online (will probably order from Forza AudioWorks even though it's a bit expensive). Not expecting magic with balanced cable but perhaps
slighty more headroom?
Edit: Hmmmm, classical music flac files lossless (and playing it loud) sounding so good it may actually change my listening habits.