I don't have a specific test disk to suggest, but would note two points. Firstly, beware New Age "acoustic" guitar recordings such as those made famous by Windham Hill-type artists. These generally feature very pumped up, exagerated sonics, designed to sound cavernously huge with ultra-sustain, acheived through compression and reverb studio effects. While some folks will enjoy this sound (can't stand it meself), it is not informative speaker test material. Secondly, let me opine that I think a great and neglected genre for doing just this sort of test is flamenco guitar music. Guitar in general is not terribly demanding for a speaker in many ways (not possessed of the greatest range of frequencies or dynamics, doesn't have difficult duration, and must be mic'ed close, limiting spacial perspective), but it does have some good things going for it. For one, we're all familiar with what a real acoustic guitar sounds like in a real room, much as we are with piano. In addition, well-recorded nylon- or gut-string classical guitar should never sound at all "electronic" in character, something easily perceived (not so much with steel-string arch-top [jazz] or flat-top [almost everything else] acoustics). Finally, flamenco guitar in particular exceeds just about all other instruments and musics in testing one crucial quality that any speaker with audiophile "aspirations to greatness" must have in spades: Articulation - specifically through the heart of the midrange and lower treble. The combination of highly percussive attack and short decay, on both rapidly strummed passages and lightning-quick single note runs (and if there is a dancer, added to by the foot-stamps and castenets), or the machine-like intensity of right-hand tremolo technique, is incredibly revealing of any inability on the part of the speaker to get out of the way and follow the music accurately - right where it counts the most. (Sorry about not making recommendations, but all of my flamenco is on long-out-of-print LP's. I also agree with suggestions to try bluegrass music, but guitar is usually not the featured instrument, and the ring of steel strings and general cacauphony of the band will make it harder to use for pin-point diagnostics.)
