Eugene Bozza's "Children's Overture" performed by the Staff Band of the Norwegian Army is an excellent reference track - has lots of different instruments, dynamic range, etc. - perfect for A-B auditioning of hi-res DACs, tube equipment, interconnects and generally everything in the audio chain. I haven't tried out most of the others due to the size of the downloads, but I'd suggest giving a pass to the "Bridge Over Troubled Waters" track - it's a purely wind-instrument piece, not useful for testing definition or dynamics or almost anything, IMHO.