So many variables with audio

Live and learn.

Adding to the frustration is the fact that when a potential equipment purchase is auditioned, you're listening to the sum total of the system, not just the new piece. Is that sibilance or lack of midrange clarity the fault of the new speakers I'm thinking of buying or is it something else in the chain?

Yes but does not pay to be frustrated by that.   It is what it is, just one data point.  You need many to be able to draw reliable conclusions. Good things never come easy.

Sometimes yes, sometimes, no, sometimes maybe, but then maybe not.
Could be, should be, would be, but as always YMMV.

The hyperbole in your question --- "a million different answers" to a question --- distorts the issue. For example, most questions are much simpler than this and get *relatively* few reasonable answers.  It is apparently fun for some to ask "millions" of philosophical (non-audio) questions in the discussion forums, though.