You'll need now to go through the Audio MIDI setup for the device, item by item, and check what it should be. If you have volume control sliders in Audio MIDI, for example, you'll need to set those to 199% as well. Make sure the speakers are set up as two channel there as well. Since I can't see what settings you have, I can't tell if anything is wrong.
For software, you could try a trial version of Audirvana, which is an expensive product to buy but works around some of the Mac audio shortcomings. Of course, you'll just need to know how to set that up instead!
If those suggestions don't work, you may need to turn to a level matched blind test to confirm what is different. And you can search for Mac on this forum to see what other solutions people have tried and used.
There is, of course, also always the headphone out socket to try with a converter to RCA and see what that does!
OK well it's not the cord (thank goodness)...just checked with USB-C to USB-C and no difference. Went item by item and all is as you suggested in audio-midi setup. I will look at Audirvana maybe....or spend the money on CD / ask friends for their old CDs.
But let me put it this way...if my friend (who does not have much of a musical ear)...can hear the difference...then something is definitely going on and it is no a figment of my imagination / placebo effect of new gear (actually she heard this when I had the old Sony DVD/CD player hooked up to the DAC)