any sub/sat 5.1 systems suck.....for music at any rate...lol
even M&K, KEF or Bose high end stuff doesnt compare to full size speakers.
for your car test, i'd suggest REM to do a general quality check, they normally record their albums very well, and to test extremes any Prodigy or Chemical Brothers or suchlike will work, massive bass swings and plenty of high pitched stuff with extremely fast beat rates in some cases, perfect for testing a speakers constraints....and in the case of older Prodigy/CB stuff no swearing
edit: oh and not forgetting The Orb....some of their stuff is fantastic for seeing what a speaker can do, specially their older stuff. (highly recommend the track 'A Huge Evergrowing Pulsating Brain That Rules From the Centre of the Ultraworld'.....i think i remembered that right...lol)
Classical music is very good too, however you want to get a good recording, not a cheap CD.