The human mind is much more powerful than our ears. We inherently interpret everything we hear into something we recognize, even when the sound is nowhere close to reality. Take the sound effects in movies... most common sounds in movies like the sound of a person bein g punched or the sound of a handgun being fired are nowhere close to what those things would sound like in real life. However, because we interpret all sounds, those non-real sounds seem very real to us when we watch a movie or TV show.
The brain can easily fool our minds. I used to do an experiment with audiophiles where I would swap out cables fron bhind the gear while someone listened. I would tell the person what each cable sounded like and what to listen for, and no matter what, they always heard exactly what I told then they would hear. The trick was that I never actually swapped a single cable yet the listener was 100% convinced they heard the difference.
I did another trick in my recording studio where people who wanted to get too engaged in the recording process, regardless of their ability, would get to sit down and adjust three dials which were lit up with bright lights and clearly labeled "Soar", "Thickness", and "Depth" (or something like that - the labels would change all the time). THey would spend hours getting the three controls perfect for their sound, and they would swear they got it exactly right after careful and sustained efforts. I would agree with them that it sounded great, but theknobs didn't control anything at all. They were placebo dials to let those losers add their input without actually hurting anything.
I could go on and on.
If you believe a line conditioner will make all the difference in the world, you will accept that as truth and hear that truth even if the difference isn't there. The only true test of a component's impact on the real sound is called a Blind ABX test. In those tests, the listener can switch between component A, component B, and an unknown setting of "X" which could be either A or B. The listener has to tell the tester (who also doesn't know what X is) which component X is, A or B. Then, the results are tallied and if the listener cannot accurately tell the difference more than 66% of the time, they cannot really hear a difference (if they are right only 33% of the time, that is evidence there is a difference as well).