Tubes are like light bulbs -- turning them on and off shortens their lifespan.
If you plan to listen later the same day, leave them on. I might turn off my amp if I was listening at, say, 9am and didn't plan to listen to it again until 9pm. But aside from that, you'd just be abusing the tubes by turning them off and then powering up again a few short hours later.
If you plan to listen later the same day, leave them on. I might turn off my amp if I was listening at, say, 9am and didn't plan to listen to it again until 9pm. But aside from that, you'd just be abusing the tubes by turning them off and then powering up again a few short hours later.