Is it possible a 20 amp outlet can make a big difference in SQ?

You describe the improvements you get from rectifying an inadequate power problem.  You didn't change the power cord at the same time did you?  The most common cause of this problem is an inadequately sized (say 14-16 awg) power cord.

My guess is the 15 amp outlet was poorly installed or defective.

Never use the push holes in the back of an outlet, use the screws and bend a loop in the end of the wire.

Now for electrical safety discussion: How do you know the breaker is correct? Realize that the breaker is sized to protect the wires in the wall. for a 20 amp circuit, you need 12 awg wire. Hopefully that i what you have. If you have 14 awg wire, you should put a 15 amp breaker in. No need to change the outlet other than if it has the horizontal slot it could confuse someone, but not likely in your listening room.

Jerry

@steakster You mistake the difference between electrical safety and audiophile sound.  for example, the push in stabs are not going to give you audiophile sound with a point contact but they won't get hot or burn the house down.  

@steakster I'm sure I could never impress someone who already knows it all.