How do you know what you're missing?

Q: " Without listening to better systems than your own, how do you go about targeting what to improve?"
A: Listen to live, unamplified music. H.P. Called it "The Absolute Sound".

Even the deaf  feel vibrations.....

Life music would be the best, except it depends on acoustics on the venue - from good to very bad.  What about Rock?  Most of concerts I attended had horrible acoustics.  The best seats in Symphony Orchestra are sold only to subscription, while affordable seats are often very bad.

When I had inexpensive receiver and speakers I had to use tone controls often and even with adjustments some CDs sounded horrible.  With better gear I don't have and don't need tone controls.  Most of music sounds good.  It is perhaps because my previous gear had poor frequency response, resonances, distortions etc.
Often cheap boomboxes have equalizers, not only for marketing, but also because they really need it.  So, to me better gear is the one that makes more CDs (or LPs) sound good.  Of course some music has exceptional and some has average recording, but correlation exists, IMO.

I don't think I presented the question well. I'm looking for those subtle things that happen when you change a cable or roll tubes. When doing an A/B comparison it can be fairly easy to pick out which one you like better and maybe even what about the difference you liked. When reading about A and then reading about B and then trying to compare that to what you already have things get difficult. I know it can get better, but I'd rather do more than throw money at it and hope. I want that magical ethereal quality where the music hangs there in the air, leaves your jaw slack, tears welling up in your eyes, and transfixes you as it tugs on your very soul. Maybe I'm just not affected the same way other people are. Maybe I can't ever afford a system that will do that for me. Maybe my room is so god awful I'm slogging uphill with a big bag of rocks over my shoulder. Maybe I need some mushrooms or LSD. I feel like I people are talking about 3-D IMAX and I'm flipping through a newspaper.

Is there a way to truly understand the concept of better sound without experiencing it firsthand? I've had a similar revelatory paradigm shift before, but I know there must be another level. Before spending obscene amounts of money in this pursuit I'm trying to find a road map or set a goal marker. Like I can't get from LA to NY on $14.37, but there's a big difference in cost between private jet and Amtrak. Maybe I'd be better off just settling for a post card.

Actually, I think you did present the question well.

*** How do you know what you’re missing. How do you know what is possible? ***

There will inevitably be push back to this notion, but to know what is possible or, at least, get on the right track to knowing: attend as many live performances that use little or no sound “reinforcement” in a decent acoustic space as possible. Then you’ll know what is possible. You’ll never quite get there, but at least you’ll have a better sense of what direction to go in.