It is interesting to hear how good equipment (sounds fine with good source material) will react with marginal sources. I agree with the "marginal" assessment of the Rumors disc; that's a difficult one indeed. If a rig is very highly resolving, then chances are it won't render a pleasant listening experience from poor source material, so this makes a good test. I don't want my rig to sound good with only the best inputs, because so much decent material out there is less than perfect, so I try to compromise. That means I won't get the best resolution from the best media, but I'll get more acceptable performance overall; a reasonable compromise since I have only one system vs. some people who have two, three, five! rigs etc.
Other CDs in that catagory: the old Hooters "Nervous Night", Brian Adams "Reckless" (so much energy in this great older recording that you can't sit still). On the somewhat dull side is Moody Blues "Threshold of a Dream". I use all of these when tuning my rig (tweaking with cones, pods, cables etc) & of course other material of highest quality as well. This can indeed be a tough balancing act.