Telling musicians to evaluate and choose their instruments in a “scientific” way?

How do you think this would go over?

“This mass produced guitar measures the same as your vintage Martin on my oscilloscope, so any difference you hear is just expectation bias.” “You need to do a double blind test to prove there’s a difference!” “Rosewood is rosewood, there’s no difference between this Brazilian that’s been seasoned for 20 years and that Indonesian that came off the boat a month ago, you’re being taken in!”

Frogman explores the issue far more eloquently than I ever could. And oh yeah... A few years back I happened to score a box of genuine tortoise shell guitar picks that had been sitting around unsold since they were still legal.  Yes, I am glad that the tortoise is no longer hunted down for its shell,  But the fact remains that not only do they truly make my steel string acoustic guitars sound better than a plastic one ever did, they reshape themselves over time to make themselves ever more comfortable to hold. 
Excellent analogy. 
Further, Gibson players don't denigrate Fender players for having inferior equipment. Effects pedal makers aren't accused of using snake oil because someone says they can't hear the effect, etc.
I'm not a guitar player but have dittled with some at the music store.  I can Feel the difference just holding and moving my hand upon one guitar from another.  Watching guitarist friends trying out different guitars.  Sound is first off.  Listening while strumming chords.  Tune it up.  But once past that they seem to be more interested in moving upon it to make music.  Kind of showoffy really.  Finger acrobatics. I can't imagine how objective science via an oscilloscope will show this.

This goes beyond guitars.  Piano, violin, clarinet...  Stereo system.  Years ago I went out to purchase a new french horn.  That horn of all horns for me. Got down to 4 manufactures/models costing between $8-12,000 at the time.  They all are fine horns as resale values represent.  Still fine horns by todays standards.  The one I purchased ultimately was because it was the most Friendly to me.  Did what I asked it to do.  Almost helping me play my best.  Objective scientific measure?  Not yet.

Devising effective tests for audio equipment SQ is child's play compared with doing it for the violin.  Every piece of wood is different.  MDF that most box speakers are made from is pretty much homogeneous.

However we still await a comprehensive suite of tests that can fully explain and predict SQ in audio products.  Perhaps even wire.  I believe we may be waiting some time.