Headphones that don't hurt, and just flat. R70x have fulfilled both of those for me. I headphone I can wear and listen for hours, neither my ears or head/neck get fatigued. And I never have to worry about some dumbass harmon tuning, V shape, forward sounding etc. I never think about sibilance, or too much mids or too much too little bass. It just is. Closest I've heard from Sennheiser so far is the HD58x, not the HD650, but the HD58x. And HD58x that was more refined with more width in sound, improved staging.
- Deeper ear cups similar to Focal, for supreme comfort
- Improved headphone pads that don't squish like the HD58x-HD660s series. Pads that are more resilient and keep their shape.
Also the holy grail of headphones should be one that is accessible. Currently it seems the HD650 is the holy grail among Sennheiser, because it is accessible to most people's preferences and monetarily. Holy grail shouldn't be a $3000 only for the few, then it wouldn't be a holy grail, it would just be the privileged.
- Holy grail should be a headphone that will last as well. I see lot of people come and go with the HD800 and it's iterations. But people keep returning to the HD650/HD600. They go buy their $5000, but inevitably go back to their true love. That is the holy grail, the headphone users realize at the end of their journey realize, you know what I actually like? It's the HD600, that's the one I want to reach for time and again.
- Holy grail is the headphone that doesn't market to the FOTM, it's the one that survives all the fads. The one that doesn't need to change to keep with the current trends.
But if you could make a R70x with say hmm... more detail, that would be the holy grail for me.