So many variables with audio

You get a million different answers, because of the many different experiences. I recently changed from an integrated amp to separates. My speakers sound significantly different now. So, if someone listened to my speakers with the integrated they would describe what they hear totally differently from what they hear with the separates. The fun and frustration of being an audiophile are the endless combinations that can make a system enjoyable or disappointing.

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.