Transient Response: When tested with a squarewave, some amplifiers,
and most loudspeakers overshoot and ring to a stop. With amplifiers
and CD players the ringing can be zero. Only a few loudspeaker systems
have near zero ringing. I don't know the threshold of audibility for
ringing.

Since overshoot and ringing can be near zero for amplifiers and CD
players, that should be the required standard.

I'm not sure how useful the squarewave test is with amplifiers and CD
players. Stereophile does a test on components with a 10kHz squarewave. A
component's transient response is mainly a test of it's bandwidth. The
frequency response of a 10 kHz squarewave has significant energy out to 100
kHz (10kHz fundamental, 30kHz, 50kHz, 70kHz,....). Whether the component
has the bandwidth to reproduce these harmonics seems pointless since your
ear will just filter them out anyway.