To answer your last question I should have a try with some old mono recordings. In general as a DAC gets better, it gets easier to listen to recordings (old ones are good examples) which have artifacts so perhaps I should just use Audacity to mono-ize a more modern recording and see where that takes me. Otherwise the artifacts becoming less intrusive would be a 'tell'.
When I needed to make something mono for the purpose of testing (e. g. setting levels) used a VST plugin for AIMP (my go-to player). I'm sure there's something to that effect for foobar2000 as well. Or, the simplest way could be an RCA Y-splitter.
Not that I know what to do with that information, to be honest (whether or not it can be heard in mono), but for some reason I'm thinking that critical listening for small nuances could be simpler with mono. But I may be wrong.
Not that I know what to do with that information, to be honest (whether or not it can be heard in mono), but for some reason I'm thinking that critical listening for small nuances could be simpler with mono. But I may be wrong.