Uneven soundstage help, please.

Well, to completely understand, you can’t move the speakers, and you can’t add better room acoustic treatments, like GIK Art panels which let you pick any artwork and put it on them.

Those are always my first two suggestions. You can try crossing your speakers in front of your head, minimizing side reflections.

Lastly, if none of this work it's probably becuase you are getting a very unbalanced frequency response.  Assuming it's not bad speakers, you will need to measure and adjust using EQ, either from a separate unit, or by using the built in EQ of Roon or similar. If you aren’t familiar with measurement and correction I suggest you get something that has room correction built in, like Anthem pre/receivers, etc. or a miniDSP unit with Dirac live built in.

Here's an example of how I corrected a much milder problem than you are having using the built in EQ from Roon:

https://speakermakersjourney.blogspot.com/2020/01/the-snr-1-room-response-and-roon.html

What you should do is pick a speaker with the least reflections around it as your baseline, and try to get your other speaker to match it.

If you use ARC, like Dirac or Anthem they will do it all for you.

Thanks for great suggestions. More details: it's not the amp--no problem with different speakers. Equilateral distances and height and rear wall distance all the same. One rear wall is a thin layer of veneered plywood, the other is plywood backed by the metal frame of a large oven. I have one speaker toed-in more than the other, which helps vs hinders the problem That one is facing a wall of angled picture windows across a shorter distance than the other one. which faces more complex, longer angles. The high ceiling above them is slanted differentially too. I could hardly draw a room map, it's so complex.
My source(s) are the same as before when I had my old speakers-- the weak spot in my chain. But why would they suddenly worsen the moment I set up the ATCs? And I totally trust the wonderful guy I bought the speakers from. They worked impeccably in his system.
The volume from speaker to speaker is consistent. It feels like a romm/placement deal.
Of course I'll check out all the things y'all have mentioned, as well as the products/devices.
It definitely could be my ears. I'm 68 and I know I hear a bit better in one ear than the other. It didn't occur to me that this could affect soundstage. It's just something i'm used to.
I'll check connections. I did have trouble with one of the terminals on one of the speakers originally, It was very fussy.
I can't help thinking it's at least partially things about my room that are more unfriendly to the newer speakers. They're closed boxes and the KEFs were rear ported. Maybe this makes a difference? I would think it would help with a difficult room. Any more simple placement ideas? Maybe something counterintuitive?

I just tried sitting backwards. Bingo!
What can I do if it's my ears? 

I doubt it's the porting, but it could very well be the dispersion, or how wide speakers maintain a flat response.

In a cluttered room, you want narrow dispersion.  In a big open room you want wide.

Best,

Erik