The concept of a test song is tricky. I have certain songs that I consider torture tracks for a vinyl rig (Slim Gaillard - Jump Session - 78rpm) and other songs that I feel like run the gamut of a full spectrum sound (Radiohead - Pulk-Pull). These can reveal the health of my system after a change.
On the other side of the coin are those songs you know so well that any change in soundstage or presence can be detected. Dinah Washington - Blue Skies (33rpm 10" Jam Session - Mono) and Red Allen / Coleman Hawkins - Summertime (12" - Stereo) are my goto's in that department.
Finally, as some have mentioned here, songs to show off your system to others. I have totally given up on this concept - bring whatever crap you want and I'll play it. My musical passions of the week (Teenage Jesus and the Jerks, anybody?) just turn others off.
