If you're looking at classical, here's a few to try:
-Gustav Holst - The Planets suite (the
Dutoit / OSM version is my personal favorite)
-Johann Sebastian Bach -
The Four Great Toccatas and Fugues (performed by E. Power Biggs)
-Johann Sebastian Bach -
Bach: Violin Concertos (out of dozens of recordings, this one is a personal favorite and sounds like maple syrup on nice headphones)
-Gabriel Faure - Requiem Op. 48 (there are many recordings; I like
this one)
-Felix Mendelssohn - Violin Concerto Op. 64 (
Itzhak Perlman's version is pretty nice)
I picked these particular selections both because I happen to like the performances and, more to the point, because they're particularly good sounding recordings, which is what you asked for. The Holst and first Bach will show off the balance and wide frequency bandwidth of the DT880, which are important with full-scale orchestral and organ music, respectively. The Holst, the first Bach, and the Faure will demonstrate the bass extension of the DT880--it goes deep, even if it's not hyped up. The second Bach, the Faure, and the Mendelssohn will demonstrate the upper midrange presence and treble extension, which are important for a realistic rendering of midrange instruments like the violin in the Bach and Mendelssohn, and the choir in the Faure.
EDIT: I forgot to mention that the Bach Toccatas and Fugues is an awesome recording for another reason. The chapel in which it was recorded has four separate organs, in four separate quadrants of the space, all controlled by a single console. The organist can play any individual organ, all at once, or any combination. The recording actually conveys spatial information about the locations of each instrument--it's not quite like a binaural recording, but if your equipment has good enough imaging, you can pick out which direction the music is coming from. You can even hear parts that sound like they're coming from somewhere just behind your shoulder, which is pretty eerie for a stereo recording.
I am SO excited. 
