Referrence music....

I have one song on CD that I play each time I change out a piece of equipment. It is Sarah Brightman singing Scarborough Fair. Anybody else use a referrence song to check for improvements, changes, etc? This is a newbie question.
Yes, a few, which I use since I've seen the artist live and since I like the music, so it's likely to get played a lot. For instance there is 'The ghost of Tom Joad', Bruce Springsteen, which has a super-realistic hi-hat crashing in. For low-end performance I use some music from bass-player Stuart Hamm, and for soundstaging I use a recording made by a friend of mine (sound-engineer) from North Sea Jazz festival. For voices I use a recoridng made by me from a barbershop-choir, and so on, and so on.... I think you can listen best with music you play a lot. It's no use buying a system because it's capable of sounding outstanding with classical if you're a Slayer-fan. Then again, a good system should play everything you throw at it.
Black Album by Metallica. It's my DSOTM. I have literally heard it on every stereo I've owned. That an I use various acoustic recordings of low brass instruments. Brass is what I am most familiar with since it is the instrument that I primarily play. So I like to see how trombone, baritone, euphonium, and tuba are rendered. Acoustic guitar does not interest me as much. But overloaded marshall stack is another matter. :-) For that I use Pantera's Live 101 Proof, Gorefest's Eindhoven Insanity, and NIN's And All That Could Have Been.
When I was auditioning new speakers a few years ago every shop I went into played Jennifer Warnes "Somewhere, somebody" from her "Hunter" CD so I've kept using it for that purpose. It also works well to convince your friends they should upgrade their own systems.
Both previous posters have excellent comments regarding using records which have the music you like and have instruments playing that you are familar with. A record I have found useful (in the extreme) in evaluating equipment as well as set up issues, especially set up issues, is called Depth of Image by Opus 3. It has a series of tracks which include vocal, solo instruments, small groups, small bands, etc and the record jacket tells you what you should hear when you play a particular band. Apart from that when I demo equipment I usually do NOT use high quality discs - if I do I just start listening to the music and forget what I came for! I have about 5 discs with borderline problems. For example one has a piano recorded too closely - on a piece of equipment with tilted up highs it becomes etched and unmusical. Same for a vocalist and the sibilance issue. I like simple recordings with multiple spaced instruments to see how well they are differentiated from each other and spaced apart. I do not use large scale orchestral demo records unless I know all about their production - these can sound good on lesser equipment and give you the wrong impression of the quality of what your going to buy.
Hi! I used to like how Metallica "Black Album" sounds, and actually still do like how it sounds in my car. But ever since my system evolved and became more resolving, I simply can't listen to this album anymore- it sounds very compressed and kind of edgy and fatiguing. Still like Metallica though. Regards.
Among many recordings, I like any of the piano trios performed by the Beaux Arts Trio. Just nice and clean violin, piano, and cello (high, mids, and mid bass) on one recording with many solo sections for each instrument.

I find that usually if the piano sounds right, everything else will fall into place. So some solo piano is a good secondary.

I agree female vocals are also on the list. Then some Tchaikovsky and Shostakovich symphonies to make sure a full orchestra does not become mud.

To top things off, Dave Brubeck's "Time Out"....
A CD that's really useful is Holly Cole's "Temptation." I will do a great job of testing how your system handles female vocals, piano, bass (acoustic) and silence.
Dire Straits, On every Street, song You and Your Friend
Excellent vocals, imaging and best example of a decay of a note on a digital format.
The "Depth of Image" LP from Opus 3 is also a favorite of mine. As Newbee comments, all of the cuts are of voice or acoustic instruments, it has a wide variety and combination of instruments, all the selections are well recorded, and they all come from different recording sessions in different acoustic environments. If you're using only digital sources, Opus 3 has compiled many of these cuts plus some different ones on their Test CD 4. WELL WORTH having on hand.

Whatever you choose to do, though, don't get trapped into listening just to a small ensemble (solo voice and a limited number of accompanying instruments, for example). You run the risk of assembling a system that will sound great on that one recording, but not on other things. And, using more complex music will tell you more about how well the system can articulate multiple instrumental lines without becoming congested. I've listened to many systems that have sounded great with a small jazz ensemble or a singer/song writer, but simply fell apart or became incredibly congested on more complex jazz or classical music.

As Sugarbrie suggests, add some well recorded large orchestral to your critical listening - it will tell you a lot very quickly once you learn to listen into the fabric of the music and not to the hyperbolic bombast of that latest great drum thwack. If I were to choose just one orchestral recording, it would be the Mercury recording of Stravinksy's Firebird (Dorati/LSO), particularly the second half. And, if I were going to evaluate a system with just one recording, this one recording would tell me virtually everything I needed to know about that system relative to my listening priorities.
I play one of my favorite songs, partly because I know it so well, partly because it contains a suitable gambit of sound quality "tells" and partly because I'm just excited to hear one of my favorite songs, but better. And then I play other songs which have different characteristics.