With tube equipment it simply has to do with the cost of retubing. Better to turn off tube equipment and warm it up for a short while prior to listening, unless you will be doing a lot of listening during the course of two or three days, such as weekends, where the thermal issues associated with turning off and on are probably harder on the tubes than leaving the unit on for such a period. This last recommendation comes from one of the people at ARC who provides information over the phone.
