When evaluating audio equipment and speakers by ear ...
How do you know what you perceive as "better", is really better?
Is what you're hearing a more accurate reproduction of the recording or is it one modified to your liking. Your ear-brain can say: "these guitars sound real", "this system gave this recording a presence I've never heard before" or "this speaker sounds better than that one". But can you tell if the equipment it's going though is accurately presenting the recording?
I know, Floyd Toole, did double blind testing on speakers and statistically we could tell ... but that's not the way most purchase equipment nor how we evaluations things we discuss on this board. So, given that ... how can we tell by ear alone?
For example if you take a recording which is less than perfect and put it though System A and it sounds a bit harsh but though System B it sounds great ... is System B better?
Perhaps B is distorting it, such as rounding off some harshly clipped peaks in the recording (which a tube amp would do if you ran it slightly into clip) and System A is reproducing it perfectly (clipped peaks and all)
IMO this is where measurements come into play as your ear can tell you if you like the sound but can't tell you if what was put in is exactly what is coming out. Most of us evaluate speakers by taking known recordings and trying to compare on them on different systems, usually by memory. Lots of problems with that IMO ... lots of places to be fooled ...
If System B is your reference ... what does that say for all that is evaluated against it.
What does that say for all your carefully picked reference material.
Just putting it out there ... (let the fun begin)
