Electrostats and ML in general will be hard to better for your purposes I suspect.
I'd give careful scrutiny to the digital source, the DAC, and any potential issues iwth higher levels of jitter than might be possible otherwise.
JItter is usually the prime culprit when modern digital becomes outright irritating. High frequencies are where most of the nastiness occurs!
How old is the CAL DAC?
DAC technology has improved significantly over the last 10 years or so. I do not know for sure but you might be able to improve their.
AN inexpensive test would be to try a good quality used tube DAC with the current MLs, if still needed once they are in good working order.
I'd like an mhdt Paradisea DAC for your application. Very musical and no harshness ever that I have experienced over several years and various system configurations. It uses a single inexpensive tube that can be rolled to tweak the sound to your hearts content. I'd recommmend any mhdt DAC you can land though. They are inexpensive and quite top notch from my experience. I also have a SS Constantine which is also top notch but delivers a more typical SS presentation albeit not harsh at all in a good setup.
If that fails and you can attribute any treble issues to the MLs (I find that hard to accept) then perhaps try a change there, even newer MLs perhaps. Treble in modern MLs I have heard in good working order in a good setup are absolute top notch as one would expect with a good quality ES speaker.
IF a good ML based system causes listening fatigue, that might be an indicator of very sensitive hearing. Best solution might be a speaker that has limited high end extension period. Or maybe some kind of low pass filter in teh circuit somewhere, although most audiophiles would probably scoff at such a thing. Each of us hear differently though so hard to say what works best in each case.
When I was younger and could hear clearly up to 20Khz, my ears were very easily offended, even with good sound reproduction. Not as much anymore at age 50+.
Another thing: do what you can to physically and electronically isolate components from each other. More distance between components and from external devices that emit EM fields usually helps keep noise that may be irritating even at a subconscious level to a minimum. Also power conditioning to help clean the electricity up at the source for any digital source gear and also pre-amp (not power amp) could probably only help. A good power conditioner/supply strip for that application should not cost more than a few hundred max. That could be a good insurance policy at a minimum, depending on how "clean" your wall power is (varies greatly case to case).
