Wow someone mentioned Tantric, completely forgot about them, but I bet it would be an excellent midbass test, the recording quality of their acoustic intros is very good.
As odd as it is, I have come to realize that, more often than not, metal and hardcore music is one of the best indicators of system quality. Reason:Sometimes even cheap systems can make slow paced songs with a narrower frequency range sound good. But, it takes a NICE system to make something with a HUGE range of frequencies all being played at the same time sound good.
I have never encountered a system that could make quality metal and hardcore sound good, but fail with slower paced music; however, the inverse happens quite frequently.
My personal favorite relaxing/non-metal test songs (song title, not albums):
Black Sabbath- N.I.B. (amazing for higher range testing. Also, if the intro sounds good, you have a nice sub stage; its a distorted bass guitar with some VERY low frequencies)
Filter- God **** Me (awesome quality, great test of everything)
Filter- The Missing (same as above but not as much mid, very relaxing)
Ill Nino - With You (soft vocals, lots of spanish guitar, not much high end, but its great for the more mellow ranges)
Favorite 'more hardcore' stuff for testing:
-Haventt listened to much lately, and recording quality usually isn't as good. Im looking around for some examples now.
A metal instrumental would be best so you arnt distracted by the lyrics, but I cant find any.
Slipknot- Three Nill (Just the first minute and a half or so).
Sum 41- The Bitter End (pretty wide frequency range, not always holistic though)
System of A Down- Bubbles (wide range, don't have to listen to the whole thing, it gets repetitive)