Many quality preamps have been designed to remain powered on, in some fashion. I would check on CD, etc, players as many have standby features. I always liked to have mine playing well before I listened. However, exceptional power conditioning for my CD player has made such a dramatic difference to sound quality I'm now ok with the sound from standby, and haven't felt the need to further evaluate. The conditioning resulted in spectacular improvement; far more than any warm up. With SS amps manufacturers often have a recommendation. Don't forget all items that heat up also wear out/degrade over time.