Why no “Break in” period?

That's OK Doug. You hang back and I'll light the fire.
Certain mechanical devices like cartridges and speakers do indeed have  break in periods. 
All of our central nervous systems exhibit a characteristic called accommodation. All of you have done it numerous times under a plethora of circumstances. Your ears accommodate to loud volumes. Your sense of smell accommodates to noxious odors. Your vision accommodates to lighting. You even accommodate emotionally. Time heals all wounds.
Electronics do not have a break in period. You are only accommodating to the sound of your system. Your first impression is the right one.
As for Paul McGowen, he will tell you what ever it takes to sell his equipment. He is not Ralph Karsten, Nelson Pass or John Curl. You have to take whatever he says with a grain of salt. As for PS audio, their designs have gotten better over the years and you can not argue with the business model.   

Time heals all wounds.

Not true.

accommodation

Never heard the word used in this way. Is this a medical term?

Electronics do not have a break in period

Either everyone is undergoing neural "accommodation" or your statement is incorrect.

Line conditioners have a break in period after that they sound more dynamic and open.

Electronics do not have a break in period. You are only accommodating to the sound of your system. Your first impression is the right one.


As for Paul McGowen, he will tell you what ever it takes to sell his equipment. He is not Ralph Karsten, Nelson Pass or John Curl. You have to take whatever he says with a grain of salt.

Nelson Pass, John Curl, and Ralph Karsten all believe in equipment break-in, burn-in, or what ever you want to call it. The late Charles Hansen did as well.

Nelson Pass, John Curl, and Ralph Karsten all believe in equipment break-in, burn-in, or what ever you want to call it. The late Charles Hansen did as well.

The point wouldn't be that some electronics designers think AUDIBLE break in occurs (note the capitalized word),  but what evidence they have for the claim.

Do they have objective measurements showing the change


and

Do they have tests correlating the objective changes with their audible consequences, that control for well known listener biases?


If not, it's just more of the same audiophile anecdotes, unfortunately.