For most impressions I listen to a rock piece out of Japan: "Kimi no Shranai Monogatari" by supercell. Like I said, it's got a good range to it with a lot of the elements I commonly find in music I listen to, and I went through a phase where I listened to that song almost every other day. If I want to see how a rig does with something mixed really hot and bright, I use "Utauyo!!! Miracle" from K-On!! because it's stupidly fast and almost any of the songs out of K-On! seem to be harsh. Sachu was briefly exposed to the latter song, but he had to bow out about a quarter way through the song because I think it hurt his brain.
I've also tried using Copland's "Appalachian Spring" since I'm also very familiar with that piece, but the volume is too varied and meets are really not conducive to critical listening...especially to pieces like that. There's some other CDs I'm considering using more to give me more range and options, but I'm all new to this so I'm basically making a lot up as I go.