Jumping in on the first Ether CX drop was a bit of a leap of faith for me. I wasn't a huge fan of the original Ether C (pre-Flow) when I heard it in 2016 at Axpona. I had purchased the original open Ether the prior year and was thrilled with them, so I was surprised when I wasn't as impressed by the original Ether C . That being said, I have been through the full evolution of Ether tuning and upgrades with my open set (original 1.1 tuning, the Flow upgrade, and now Flow 1.1 with their new pads), so I had faith that nearly 3 years of experience tuning the C would probably end up being a sound I liked - and it is!
I currently have about 140 hours on my Ether CX, so I'm right in the middle of the recommended 100-200 hours for burn-in. I suspect they may continue to improve and open up over time, but I am already really happy with the sound considering the cost and the fact they are a closed back headphone. I purchased them to replace my Mahogany Fostex TH-X00 which I found fatiguing to listen to for extended periods. The X00 is a "fun" headphone for shorter sessions, and produces an incredible quantity of bass, but it has a bit of glare in the upper registers - plus the soundstage is pretty much the middle of your head. The Ether CX outperforms the X00 in every area with the exception of efficiency and bass quantity (but the bass the Ether CX has is better quality). The Ether CX has a vastly wider soundstage, is smoother, more balanced, more resolving, and more comfortable to wear.
Comparing the Ether CX to my Ether Flow Open 1.1 (EFO), the EFO has an even wider soundstage, better imaging, and is smoother - but the overall voicing between the two is similar. The EFO is definitely the better of the two, but that's saying a lot considering the cost difference and the fact that the Ether CX is a closed headphone. I did the Ether CX vs EFO comparison with both using the DUM cable that came with my EFO which, by the way, is a nice step up from the included DUMMER cable that comes with the Ether CX. I also had the opportunity to do a quick comparison between the Ether CX and my buddy's Focal Elegia. Both are great closed headphones, but I found the Elegia a bit clinical sounding while the Ether CX just disappears and lets you focus on the music rather than the headphone. If anything the Elegia reminds me of what I remember the original Ether C sounding like a few years back - a bit bright and lacking in bass.
As far as amplification goes the Ether CX definitely scales well with better power. My "transportable" amp/DAC is a Fiio K3 and I am using a MrSpeakers 2.5mm balanced DUM cable with it. The K3 has plenty of power to drive the Ether CX to adequate listening levels, but you can tell that it really doesn't have a firm hold on the drivers. Moving up to my THX AAA 789 definitely resolves this... The Ether CX really opens up and comes alive on the 789. Going all the way up to my Liquid Platinum is more of the same - with the most noticeable improvement being significantly wider soundstage and more precise imaging. The 789 is a good pairing, but the Liquid Platinum is a meaningful and significant improvement for the Ether CX.
In terms of negatives, the only one that really stands out to me is the packaging and box. It is minimal to say the least. My $200 HD 6XX has a beautiful box and others at a similar price point to the Ether CX, such as the Elex, also have much nicer boxes. I get that the box will likely sit in a closet or basement for most folks after they get the headphones, but still. I also wonder if the few reported issues of damage are due to the minimal packaging and box? I would deduct half a star for that if I could, but it isn't worth it in the overall review of these.
In summary, Massdrop was looking for a flagship closed headphone with the Ether CX and they got it. Give them the proper break-in time that all planars benefit from as well as decent amplification and you will probably be really happy with the sound. If you want a closed headphone that doesn't sound like a closed headphone, is pretty much neutral, light enough to be worn for hours, and pretty much disappears into the music, these are a great choice!