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

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 ... :)

I can't make any sense out of buying or tweaking a system by using poor source material. In fact, how can that not lead to big, big mistakes? Especially with speakers! IMHO, a system should be able to reproduce as accurately as possible what is on the source. (Is that not a no-brainer?) It would seem to be more logical to have additional sources, (software when available, or hardware when necessary,) to choose alternative distortions of the presentation to make it more listenable. Maybe digital equilization will be made more affordable soon that might allow favorable tweaking of the sound for those unpleasant productions. Charlie

I'm generally in agreement with Charlie, but if you're looking for a bright recording check out Gaucho by Steely Dan, especially the title track--absolutely painful.

Tim