How do you know when a stereo sounds good?

There are certainly other hobby's where people use less than objective criteria to reach conclusions about what is "good". Wine drinkers for instance. People who buy art.

Perfectly valid way to decide how to spend your money.

When you want to turn the volume up.

+1 @bishop148. Very true!  Did we miss that one somehow?

I will tell you that listening to the Phish concert recorded in the Berkeley Greek Theater in 2010 sounded pretty damn good on my stereo last night. Interleaving the songs between Quboz and Tidal MQA was really instructive in showing how much better the Quboz stream sounded!

When the stylus hits the groove of my favorite Howlin’ Wolf album, and my dog starts wagging his tail feathers. 
 For digital, that’d be Steppenwolf.