First don't use a single CD or record for your reference. As your system gets better every CD will sound different. Some better and some worse so in auditioning new audio gear listen to a lot of music. Unfortunately sometimes you don't hear all the good and bad until you live with the sound a while. I've heard systems that sound "better" then my current system and brought a component home to fine the "better" also add a lot of listening fatigue. Be careful and get parts home to listen to them one component at a time.
What s Your Frame of Reference?
Whenever I make a change to my system I pull out a select few records to evaluate the "upgrade". Gross differences in sound quality are fairly easily judged, but most likely the change in sound quality is subtle and better judged over an extended listening period. This is my problem: let's say I change something and on one of my reference records the trumpet now sounds a little more brash and upfront, maybe even bordering on harsh. How do I know whether the upgraded system is more accurately portraying the sound of the recorded trumpet, or has the upgrade merely added an upper midrange resonance problem? I have a good idea of what a generic trumpet (w/ and w/o mute) sounds like, but I wasn't at the recording session. The studio, the mic, EQ, recording medium, etc. all add an enormous amount of variables to what is actually recorded onto the record. If I judge the sound to be harsh and make changes to my system to remove the harshness, then maybe all I've done is make the system more pleasant, euphonic, but less accurate. The "live music in real space" paradigm is not particularly useful in that the overwhelming majority of the music I listen to is not of this type. Besides, the transparency of the audio engineering is still a variable. Ideally, I need a wide bandwith recording where I was present at the recording and which the engineer faithfully recorded the music. Unfortunately, I don't have such a recording. How are other dealing with this issue?