Easily becomes your favorite headphone!
I spent nearly a year on the fence about buying the Focal Elex because I enjoyed my HD6XX so much that I questioned how much better sound could get. I finally became bored enough with the Sennheisers and purchased the Focals with absolutely no regret.
Personally, I think the HD6XX is a more comfortable headphone. It's much lighter and fits better around the crown of the head. The Elex is a close second in comfort. The additional weight is well distributed throughout the headband and the ear pads. The clamp force is more than I thought it would be considering how "open" they look on many pictures but they don't stretch easily, which is a consideration if you have a bigger head size.
These feel like they're worth their price. Everything from the unboxing experience to the feel and materials of the headphone screams premium. I'm very happy in this department. No creaking while wearing them and I feel like they could survive accidental drops.
I can hear why they're advertised as "super HD650s". The sound signature is similar, with better treble sparkle. I was worried that they were going to be sibilant because I am particularly sensitive but I have yet to hear any! Instead, the treble is perfectly extended, adds a lot of detail to instruments and voices, and brings a whole new light to my music. The treble is my favorite and most surprising part of the headphone. Huge improvement over HD6XX.
The mids are similar sounding to the HD6XX but imagine that the singer moved back 6 inches from the microphone. I found that in some songs the HD6XX could be shouty but this is corrected in the Elex and makes the Elex sound like a smoother headphone to me. Overall, mids are detailed and accurate, definitely an improvement over HD6XX but I can see how some may prefer Sennheiser's presentation.
Bass is accurate, fast, detailed, but can be underwhelming depending on the genre of music. I was initially disappointed because I've read how it's well extended and flat. I think that the treble and mid presence is so forward that the bass takes a back seat. It's present, and when comparing directly to the HD6XX, the bass is stronger on the Elex but it doesn't "feel" that way initially. After a week of daily listening to the Elex, my brain has adapted and I appreciate it now but I wish the mid-bass was boosted by 2-3db.
This headphone does frequency separation incredibly well. In part due to the distance that the drivers sit from the ear and due to the openness of the design. In comparison, the HD6XX begins to sound claustrophobic and congested. The HD6XX remains a well-detailed headphone but when you listen to the Elex, it's a big step in the right direction. The Elex makes music far more engaging and fun to listen to.
The dynamics are INCREDIBLE. Also, one of my favorite parts of this headphone in addition to the treble. If you listen to jazz or classical where dynamics are an important part of the piece, this headphone will bring you to a new level of euphoria. If you're listening to poorly mastered new music, in particular pop or rock, the dynamics make no difference. The dynamics add a new level of detail to well-mastered music that I haven't experienced before.
The soundstage is better than the HD6XX but only marginally. It's marginally wider with better 3-dimensional representation but I expected more of an improvement. On the other hand, imaging is significantly improved and again, adds a new layer of detail to music. The sound is very natural and a pleasure to do critical listening but also lends itself well to laid back enjoyment.
As you can probably tell by now, I think the Elex does most things better than the HD6XX. In fact, the Elex reminds me a bit of an open-back version of the Audio-Technica MSR7. The Elex has better soundstage, imaging, frequency separation, but the overall technicality is similar to the MSR7, in my opinion. The MSR7s have sibilance and metallic treble, which is why I didn't like them but the Elex fixes those issues.
Are the Elex 3+ times better than the HD6XX? Absolutely not. Am I happy paying $600-700 for them? Without a doubt. If you have the budget and the interest in upgrading, these are without question a good value and a contender for end-game sound quality.
Setup: Tidal HIFI/Masters -> iFi iDSD Nano BL -> Elex
Good comfort, better sound, end-game contender. Better than HD6XX in most aspects. Pleasant in critical listening situations but also a pleasure to relax with. Can be your one and only headphone.
Would recommend to a friend.