Hrm, there are probably plenty of different CDs out there but what I would recommend is getting a CD or FLAC file of a song you listen to alot and know very well then listen critically. If the speakers are better than you are used to for that piece of music then you will probably hear things you haven't heard before. Thats usually how I test speakers, tho I will admit that yes, metal isnt usually the best music to test a speakers quality with.
If you want to know what I would take with me to a speaker shop to audition speakers with I would say.
Delerium - Poem specifically tracks 1 and 4.
Nine Inch Nails - The Fragile, tracks 7 off of either cd.
Dave Matthews - Crash, Say Goodbye, I think it is song 6 Tho Dave tends to make everything sound pretty good
Billy Joel - Greatest Hits 3, Baby Grand duet with Ray Charles.
But these are just my personal preferences. You might find them worthless.