@hilde45 Thanks for starting this thread. You've gotten some good stuff here from the AG all-stars. I think I know what I am doing on auditioning speakers but I'm going to be rereading this the next time I audition any equipment. A couple of things I would add.
You have to know your own ears. You have to know what kinds of aberrations that you can hear right away and what types of recordings can reveal those aberrations.
For me, solo piano gets very quickly to about 80% of what I need to hear (or not hear) in an audition. I've been able to reject some pricey speakers 30 seconds into a piano recording. It reveals quickly speaker problems that may take a while for me to discern using other music. For that reason, if I am auditioning in a brick and mortar setting my first recording is piano. Other people may do better with other types of music. Know your ears!
Massed strings has been mentioned. To be sure, a speaker that gets this right is a keeper. This is a very high hurdle. It is what you get for 80K that you can't get for 10K.
A few more words on French Horn. A good system, especially a tube based system, can impart a richness and fullness to the horn that will make you melt into a pool of mush in your chair. It's hard to describe, other than to say I am convinced that is how French Horns sound in heaven.
Certainly, voice is an imperative, and speaker manufacturers know this. It is a lower hurdle for the manufacturer. There are plenty of speakers that are credible on voice that I would not be happy with in my system.