Would be interesting to see a comparison with the 600 and 800S. Seems like everyone has different takes on where the Clear sits with the two.
Here's a comparison from AB'ing the clear and hd600. Just wanna be transparent about my setup: my amp is an acro l1000, via its 4 pin balanced out for both headphones. It is a fairly neutral/clean dac and amp combo by my tastes (only direct "A/B" was with the questyle cma 400i; it made the questyle sound muddy in comparison. I also wanna add that the amp definitely does not have any "sabre glare" from experience with this occurrence). As far as I know it is fully balanced, with 2 internal AK dacs and all.
I think the clear is overall a warmer headphone, but there is definitely more treble peaks or sparkle - whichever way you wanna spin it. The two most contrasting aspects to the sound is the sub-bass and the upper mids and treble, but I think I can hear the metallic quality talked about on the clear.
Difference is sub-bass quantity and impact is tremendous. In perfect honesty, I would liken the difference between the two to adding a subwoofer to your speaker setup; it is that substantial. Bass guitars, acoustic guitars, and kick drums sound have more impact and much more body or reverberation, particularly kick drums. If your a fan of an analytical sound, or really used to the hd600, I wanna say the clear can sound a bit too aggressive, maybe even "boomy" or muddy (mostly referring to kick drums; acoustic guitars are just plain better to me on the clears). It does bring in the question of realism though; I don't have much experience attending live performances, so I wonder if the clears presentation is more realistic here. Maybe someone can comment? I also wanna add that I think listening to the clear balanced vs single ended made the bass more impactful, or maybe just extended the bass more (?), but take this with a HEAVY grain of salt.
I don't think the upper mids are much more recessed than on the hd600. I love the hd600's very forward upper mids, and I'm pretty reliable when it comes to complaining about faults in this area. Female voices do sound a bit more distant (very slightly). I think its the soundstage/imaging difference? Female voices are noticeably smoother on the hd600, but funny enough male vocals sound slightly harsher on the hd600 than they do on the clear. The clear is definitely more prone to whistling/raspyness/edginess in female high notes, and these voices in general are "colder" sounding (not a slight change, it is very noticeable). I wanna say I can hear the metallic quality of the clear here. Again though, I think there is a difference between using the clears balanced vs single ended, with the edge being taken off a noticeable bit via single ended. I feel more confident with this than the note on the bass earlier. For what it is worth, in the past I gathered from some impressions of my amp that the single ended out was slightly warmer than the balanced out, and I have generally come to agree with this over the years.
High notes on instruments like electric guitars and cymbals have more edge/spark on the clear.
Soundstage is slightly wider on the clears - key word slightly. Not too sure about depth and/or height. Imaging and/or separation is better on the clears. Very 3d or open is the impression I get compared to the hd600, but I'm struggling to find the right words here. Again, I think there is a difference between balanced and single ended, with the separation or imaging sound a bit better via single ended on the clear, which is something I never experienced before. Maybe because things are not as wide sounding (generally the main difference I hear with balanced)? Again, take with a grain of salt, but I feel pretty confident with this.
I think the clears sound better, for my tastes, from the bass to the lower mids, and the hd600 still sound better to me higher than that (they are my favorite headphone ever). The clears do get closer to the hd600's past the low mids unbalanced though, I'll need to confirm the differences described however. I really like that the clears do not screw up the upper mids, despite the metallic tone/quality I think I'm hearing up there. I think this will be more of a problem for people who listen at really high volumes or are very treble sensitive. On that note, I think the clears are a really great headphone for listening at low volumes! Plenty of bass impact, sub-bass extension, and mids are not recessed.
I would label the clear as pretty aggressive sounding compared to the hd600 just going off the bass impact. This, with the difference in tone in the upper mids, make me hesitate to say its the best upgrade from the 600, which seems to be a thing online. I can see why people say this however. As a 600 lover, the clears do sound pretty great to me, definitely more than the hd800S and the audeze lcd-gx (I know my comparison didn't seem like this was the case, but I focused on describing the differences). YMMV based on how sensitive you are to the metallic tone thing if you also love the 600's, which will be the biggest hurdle in my opinion.
Hope this was informative! I really rely on comparisons myself when shopping around so tried to be really detailed without too much audiophile lingo. I'll compare against with the hd800S eventually.
Pics so you know I'm not full of it