How do you know when a stereo sounds good?

How about this it’s very simple and you don’t need 

all the bullshit theory’s or work for NASA. 
It sounds good when ever you think it sounds good. 
It’s your music it’s your place of living it’s your stereo 

when you think it sounds good that’s all that matters 

there it is plain and simple 

When you turn in on and listen...its trilling.When you get an album by an artist you like and play it and say WOW that sounds great....

When you play a album and the artist or group sound like they are playing live ,in your room....

When you stop talking about the sound and start talking about the music.

Well, if any question begs a tautological response, this is it.

A stereo sounds good to me, when it sounds good to me.

A stereo sounds good to you, when it sounds good to you.

It doesn't matter how one interprets the questions posed (whether "you" was intended to mean "one", or "me" as I read the question, the answer is the same). I've heard any number of systems that the owner was enthralled with, but sounded pitiful to me. In none of those instances was either of us wrong.

If the questions were intended to ask for a differential comparison, that's a very different matter, but also subject to the same preferential differences.

I would say, however, that if you cannot enjoy good music on even a mediocre system, it's not just the music you're interested in, or you have a very different understanding of music than I.  I fully enjoy listening to music in my Westy Vanagon, although it has the acoustics of an F2 tornado at highway speeds. I've flown >1M miles listening to Bose (or worse) headphones in horrendously noisy aluminum tubes, and enjoyed thousands of hours of music. I feel sorry for folks who can't enjoy the beauty of music if the decay of a cymbal isn't perfectly reproduced, or the "midrange" isn't perfectly liquid. Is it *more* enjoyable with better sound in a better acoustic? Sure, it can be, that is why we're here, theoretically.

But, I can be, and have been, brought to tears on particularly beautiful and soaring female vocal lines with mid-level cans on Youtube. If that's impossible for you, then I think you're missing an important part of the beauty of music. Mediocre reproduction of beautiful music is still beautiful. Beautiful reproduction of crap music is still crap.

That's just my opinion, of course, and it's totally impervious to the approbation or denigration by other's differing opinions.