How do you know when a stereo sounds good?

Sorry but this phantom image between speakers is not even the beginning of acoustic control...The stereo sound must be out of the speakers and filling the room or around you and even coming from back of your head in some recording....All recording albums are different, but if the sound is ONLY between speakers, your stereo system is not optimized by acoustic at all....

Your room is probably under controlled...

Keeping it simple when the sound has a strong phantom center image and my speakers seem to mostly disappear.

When my brain gets tricked by where the sound is coming form.

When you find yourself thinking back to how great Led Zeppelin and other former favorites used to sound so good and instead you are listening to Steely Dan who you never gave much thought to previously.

Bottom line; when you find yourself much preferring artists that you never know existed before you spent 20k or similar putting a nice system together.

@mahgister Of course all that too.  I'm just describing the beginning of what I find enjoyable with a well implemented audio system.

When I close my eye doze off and feel Dianna Krall strum banjo but wake and it really the maid with feet.

When it doesnt need Synergistic Research and similar junk magic props.