How do you know when a stereo sounds good?

I listen and the music draws me in to stay.

What Mapman said. Plus:

• There is some magic between the speakers- a heavenly illumination .
• My wife sings more.
• More genres are enjoyed.
• When I still tinker but more to explore possibilities than to look for change.

One other thing. Subjectivity fits my needs as a curious person, experiential learning works best for me. I don't like biblical cannons feeding me the truth.

When is the water warm enough for me to take a comfortable shower? I guess you just know.

@elliottbnewcombjr ,

"when the music becomes so INVOLVING you forget about the equipment, get lost is the experience"

The same for me.

It's a similar experience as watching an involving film or reading a gripping novel.

All of which require a willing suspension of disbelief.

The exact point at which this occurs will, naturally enough, vary from recording to recording, system to system, and individual mood to individual mood as anyone familiar with the Edison tone tests of the 1920s will appreciate.

https://phonographia.com/Factola/Edison%20Tone%20Tests.htm

You first have to know what a good system sounds like. You do that by auditioning the very top performing loudspeakers and equipment. Find the sound you like, then move down the product line until you can afford the product and still gives you a taste of the best systems.