Bring your own music that you are very familiar with.

I bring these 5:

enough is enough duet, Barbra Streisand and Donna summer, hear their voices distinctly?

https://www.discogs.com/Barbra-Streisand-Donna-Summer-No-More-Tears-Enough-Is-Enough/master/74336

Jeff Wayne's War of the Worlds. Richard Burton's voice sound correct?

https://www.discogs.com/Jeff-Wayne-Jeff-Waynes-Musical-Version-Of-The-War-Of-The-Worlds/master/71524

Double Bass, two Jazz Bass Players. Can speakers both Produce, and Control bass?

https://www.discogs.com/Niels-Henning-%C3%98rsted-Pedersen-Sam-Jones-Double-Bass/master/56219

Cassandra Wilson, Blue Light 'Til Dawn. Her voice, amazing musicians, imaging, .....

https://www.discogs.com/Cassandra-Wilson-Blue-Light-Til-Dawn/master/40901

finally, Drums, Mickey Hart, Planet Drum

https://www.discogs.com/Mickey-Hart-Planet-Drum/release/1935048
There were a few vinyl records made in the 1970's by Lincoln Mayorga that were direct-to-disc to enhance the dynamic range and recording accuracy of the instruments.

We always used these in my shop back then to demo Audio Research and Magnepan systems, which were the best in those days.  We also recorded them onto a Nakamichi 1000 and played it through the same system to hear the differences between the original vinyl and the cassette tape (Their Dragon was a later invention.)

As for regular vinyl, we used Maria Muldaur's "Midnight at the Oasis" song and, as it was VERY POPULAR with the younger set, Pink Floyd's "Money" as well.  Customers really liked that one, although I was not that impressed with the recording.  The fabulous singer Linda Ronstadt was also a good choice.  Back then, female voices were used as a kind of benchmark for listening.

Orchestral sounds were mainly London Decca vinyl versions of whatever we had on hand.  Customers usually brought their own stuff if they were serious about wanting to listen to different set-ups. 

A local competitor/dealer, Peter McGrath, used a Stellavox and some good microphones and went around recording live music that he used in his shop in Coral Gables.  He became a bit famous for the quality of his recordings for a while, and they were pretty good!

As for digital, it was not invented yet and today, when I listen to my CD's, I am always going back to vinyl for what I believe is more accurate playbacks.  I guess it depends on what you are used to.

Cheers!
Simply listen to things you're familiar with that have at least a modicum of dynamics, and remember to work the word "modicum" into statements as people will think, "Hey, that guy's real smart."
One song I always use is "it's oh so quiet" by Bjork, i think its on her Post record.