Being a HUGE Gentle Giant fan, I have often used the Octopus CD, along with Masque by Brand X, and a few others to audition audio equipment. I have always tried to purchase gear that was the most accurate, revealing, transparent, etc. I think if you have some good recordings of piano, acoustic guitar, drums, cymbals, violin, brass, concentrate on equipment that makes all of this sound natural. Then poorly recorded music or electric instruments will be as good as they can be. You don't have to design a system around Prog or Classical, as mentioned, stress accuracy and you'll be fine.
Sonny
Sonny