There are different opinions on this. Some amp makers say do not run your amp for more than 8 hours at a time, whereas others say it makes no difference how long you run your amp. But, to answer your question about "burning in", you do need to run the tubes to get them to sound their best. You do need to run sound them through the headphones to establish a load in the circuit. As long a music is playing, even at low volume, you are burning your tubes. Tubes do sound better as they burn in, but it doesn't take super long to burn in tubes. I've had tubes open up nicely after 5 hours or so, whereas some can take up to 25 hours. But, I don't let them burn without me actually listening to music. Tubes have a finite life, so I'd rather listen to the tubes burn in than have them burn up with no ears attached to the headphones. Burn in does help, but don't go crazy with it. Just enjoy your music.
