Break In Time..just being on...or music played ?

Yes the electronic equipment is broken in by just being left on.
It's usually the capacitors that need a bit of 'refining' and they get it by just being on..
Ditto wiring,etc.

I believe it needs music playing through it. Yes, I think it's mainly the capacitors, but they need to be cycling (storing and releasing energy) to break in.

I just had a great experience using a Granite Audio MC phono burn-in disc to burn in my new phono preamp. I would not have believed the results had I not heard it for myself.

I wouldn't think that you'd need to turn your amp on, just put a cd in and have it repeat through the preamp for a couple of days. That should do wonders. I've found that it makes a bigger difference depending on how many, and how large the caps are. Usually the more caps the more burn-in required, the bigger the sonic difference.

Cheers,
John

Elizabeth is right.

The electrolytic capacitors have the largest value changes throughout the time hence need to be settled for the proper value.

The circuitry of an amplifier or any electronic device is being built the way that electrolytics will reach their certain expected value after a certain initial time of a powered standby...

The electrolytic caps define the values of the rms voltage applied at the amplification stages(or initial offset) hence carry the main importance in the time to settle.

The signal path circuit elements such as coupling capacitors, input resistors, small signal ICs or transistors will have a negligable differences compared to the electrolytics.

Jmcgrogan2...Is it also necessary for someone to be listening?

Eldartford, no, IMHO.
That's why I said that turning the amp on was optional.
He could just run the cd through the preamp, with the amp off.

When I was burning in my phono stage, I played the Granite Audio CD on repeat 24/7, when I was not listening to vinyl (amp and preamp were off). Obviously when I was listening, I listened to the cartridge with the amp and preamp on. That's how I could tell how much it opened up, even after just a couple of days of running the cd.
FWIW, the phono preamp I bought was the BAT VK-P10SE w/ Super-Pak. Those huge oil-can caps take quite a few days to burn in right. It was amazing how it sounded after a week of burn-in compared to right out of the box. Night and day, so to speak.

John