Two ways. One for digital and another for analog for me. For digital I play a Song at the level I like on the original equipment. I then play a test tone with my test cd, i measure the level of the test tone, I the swap the piece i’m comparing. I play the same test tone and again measure the level and set it at exactly the level as before. I make a note of where the levels were set on the pre-amp for each piece and then A/B listen,
the key is knowing where on the Pre-amp to set so the levels are the same. The only way to really get that right is with test tones.
People can and off times do misinterpret level/volume differences in equipment as being better or worse, or, if things are heard at different levels one may hear more detail on one piece and think that piece is better when it was just turned up louder, apples to apples, make sure the levels are matched first.
Same test for analog, just use a test tone from a record and measure and match the levels.
Enjoy
Enjoy