I agree NIN is recorded great.
FULL range, clean, tight, and while a lot is electronic there's usually some 'real' instruments in most songs. People dismiss NIN sometimes as being all electronic which isn't true at all.
And while Reznor's voice isn't the best in the world, it's a good 'typical' range, and recorded really well.
My main 'ref.' CD is Jeff Buckley's "Songs from My Sweetheart the Drunk".
I think I'd call this the very best sounding/recorded disc I own.
Fantastic recording, and fantastic music. If Robert Plant being in a band that's kinda/sorts like U2 or Sheryl Crow sounds cool to you I think you'll like this CD. Personally I love it.
Sad that is was finished after he died. He would have become very famous I bet.
His first CD "Grace" is also good, but not quite 'ref.' worthy.
Roger Waters' "Amused to Death" is excellent also. A perfect follow-up to "The Wall" (this one about the Gulf War as the Wall had much to do w/ WWII).
It's also recorded in Q-Sound and one of the best examples I've heard of how 2 speakers can sound like 5.1 (when sitting dead center).
Tori Amos's "Boys for Pele" is recorded great IMO too. Too bad her other CD's aren't recorded as well.
Great disc for female vocals w/ GREAT range and vocal dynamics and piano, etc...
Norah Jones and Diana Krall are recorded great, but IMO, they're kinda dull, and really not any kind of stress test for any system. If Tori Amos sounds great so will Diana and Norah, but not necc. vice versa.
I don't know much classical, and mostly listen to sountracks for that type of music. Of all of those Gladiator is one of the best IMO as a test ref.
Also the Princess Mononoke soundtrack, and the Red Violin. All great movies too.