I recommend Song #2 No More I Love You's from BOCA Volume 3 (http://www.casa.org/boca_info.html). It's an Acapella track, which means no instruments and voices only. This particular one is all female, and has all the voices clustered in the high end. This song really stresses the ability of your sound system to separate out the 5 or 6 different voices in the song. I've heard this song on some lower end systems and you get distortion.
For low end testing, the IASCA competition CD has a couple of "unlimited SPL" music tracks that emphasizes the low end like you wouldn't believe...