Using very bright and very dull CD's to test the extremes.

Do you mean the CD of Rumours? I find the CD transfer to be bright, glary and lacking bass. The original LP sounds MUCH smoother and warmer with more bass and a much less fatiguing top end.

For bright CDs I use Roger Daltry's "Under a Raging Moon". I love the album but it is so bright I can never get all the way through it.

Good point. "Rumours" was an analog master and can sound quite natural on good CD playback. For overly bright CDs, I use Rock/Pop recordings from the mid 80s in the early era of digital mastering.

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.

Rumours rumor. The story is that when Nautilus was remastering for their half-speed release, there was "damage" to the master tape (not caused by Nautilus) and the release was quite delayed because the master tape had to be digitized and "repaired digitally." So, who knows what the actual source was for subsequent CD masterings? It seems that a good early original vinyl pressing should be the closest to what was intended if you care to compare. Somewhat OT, but ... :)