You have the general idea, but you are wrong in inferring black and white from shades of gray!
I do believe acoustic instruments are generally the best reference material...if you are familiar with them! I once watched Dennis Murphy evaluate a pair of speakers he had never heard before. He listened to a full orchestral chord for about 45 seconds and was done! He plays in an orchestra and yes, I do believe that is helpful for his ability to evaluate a speaker so quickly.
I play in a Big Band, and have great confidence in evaluating the sound of the horns played in my band. There is variety in the sound of these horns, some from the horn brand/model, and others from the performer; however, there are enough commonalities that I feel pretty good about evaluating any acoustic instrument that is in my band. Certainly, it would be better if I was familiar with the exact performer playing the exact instrument I was listening to.
Electric guitar is a different critter. With the special effects boxes. etc. It is very difficult to know how accurately it is being reproduced. I have listened to two good speakers and while the guitar sounded different on both, it sounded very good on both, and I was at a loss to know which was correct. I figured it out listening to the horns I know well.
The reason I am specifying "good" speakers is because crappy speakers reveal themselves by having over-emphasis on a certain frequency ranges. Recordings of electric guitar or synthesizer will reveal this.
So, I am not saying only acoustic sounds evaluated by people familiar with the original sound should ever be used. However, I will agree that that may be the ideal scenario.
Hope this helps!
PS - I definitely believe it would be a mistake not to include acoustic music in your audition music.