Digital Analogue isolation /separation

I've read many of the threads on separating or isolating digital and analog AC lines. I still don't think I understand this. If the point is to eliminate digital noise from entering your analog line, does it make a difference when you listen to analog (TT) when there is no digital source playing? I guess if I am listening to digital through my analog amp and preamp I can see this but don't get it the other way??
When you use your analog I'd suggest you experiment. Try listening 1. with your CD player plugged in and running 2. CD player plugged in but not running, 3. CD player unplugged. I listen primarily to classical so would use dynamic classical music with detail, not just violins for example. I want to focus on the deep detail clarity as well as overall dynamics. I have found many classical guitar recordings can be suitable as well. To get started you need to 1st listen, with the cd player unplugged, and determine the recording you are using is in correct phase within your system. Simple to do. Listen to a portion of a cut that interests you for this analysis, then turn off your amp and wait 30 seconds. Then reverse the leads from your speaker cable at the amplifier (or at the speakers) - changing from red to red and black to black, to red to black and black to red, for each channel. Turn on the amp and wait a minute for it to stabilize. Now listen again. There will be a difference;  and the way the sound is more clear,dynamic and enjoyable/interesting is the way you should use to analyze the impact of your CD player on your analog sound. Listen at a volume that you can hear detail without straining. Let me know what you find. ATB, Peter
PS. Have you tightened the screws on your drivers recently? Enormously important for clarity and dynamics.