Both previous posters have excellent comments regarding using records which have the music you like and have instruments playing that you are familar with. A record I have found useful (in the extreme) in evaluating equipment as well as set up issues, especially set up issues, is called Depth of Image by Opus 3. It has a series of tracks which include vocal, solo instruments, small groups, small bands, etc and the record jacket tells you what you should hear when you play a particular band. Apart from that when I demo equipment I usually do NOT use high quality discs - if I do I just start listening to the music and forget what I came for! I have about 5 discs with borderline problems. For example one has a piano recorded too closely - on a piece of equipment with tilted up highs it becomes etched and unmusical. Same for a vocalist and the sibilance issue. I like simple recordings with multiple spaced instruments to see how well they are differentiated from each other and spaced apart. I do not use large scale orchestral demo records unless I know all about their production - these can sound good on lesser equipment and give you the wrong impression of the quality of what your going to buy.
