Totally agree on doing the more extended listening tests. A/B-ing is for quick impressions, not for serious listening imho.
I understand the part of reducing brightness/sharper sound with cables. While using USB this is the main reason that I have searched a long time for a decent cable. After QED, AudioQuest Coffee, Wireworld silver and platinum and many others I still wasn't satisfied. I kept on searching for the good cable and found it in Australia, linear some 10,000 miles from where I live.
Especially with the Curious Evolved USB cable brightness and/or sharpness are not a problem anymore. With this cable I experience the "softer, more mellow sound" (I'm not a Curious Cable employee, just a satisfied customer). I think this has mainly to do with good shielding in the cable.
I'm always willing to do some testing and will try optical again after reading all remarks and tips in this thread. Never too old to learn! However, what I can not explain is the worse bass (blurrier, less punch) I experience while using optical the last time I tried.