How do I break in a tube amplifier?

Why would you leave it on? Turn the amp on when you want to use it!

I do think it's important to turn it on before listening to it, although leaving it off all the time does make the tubes last longer.

To answer your question about playing music...yes .
Don't just turn it on and walk away , PLAY MUSIC !
Tubed equipment , as I have been told , need the electrical changes to aid in the break-in process .
I have not noticed a big change in electrical components resulting from break-in like I have from speakers .
And I agree about it taking @ 30 minutes to warm up for tubed equipment .
So if you only have short periods of time to listen ,tubes are the way to go . My SS phono stage needs at least 7 hours of on time for warm up and my SS amp even more .
Most people leave SS on continuously and turn tubes off when finished to save tube life .

Just another 2 cents worth .

With tubes there's a run-in period. At first, some tubes are not very quiet. With a few minutes they start sounding better and more quiet.

No offense to Polk432 but I thought it was funny how he mentioned 1001 precautions about tube amps and then closes with "Have fun. Tubes are great". LOL. This is why I went away from tubes.