I may have missed something, but I'm surprised no one has mentioned any chamber music recordings. I often listen to chamber music, and many systems which are really great on "big music" don't quite let you hear the wood in a pizzicato passage of a small ensemble. When I go to listen to a system, I bring chamber pieces where texture really matters. (Which is most chamber music, really, but for some works failure in this dimension really stands out.) Schubert is good for this, especially the C major quintet, esp esp the 2nd movement. Also the Eb major trio. Also, Shostakovitch's violin and viola sonatas. The problem with the viola sonata for this purpose is that it is in some ways painful, and so engrossing that even a mediocre system disappears and the music just takes over. Maybe that's a criteria for reference recordings - if the purpose is to find a few pieces to listen to that display the qualities of a system you are listening to for the first time, or just for evaluation, you should choose music that is not too involving, of a genre that you listen to, but pieces you can stand away from a bit. For showing off my own system, I just tell people to bring over what they like. I learn more about its flaws that way.