Tube life in preamp. Please help

Pre-amp tubes typically have a very long operating life (there are exceptions). In stand-by mode, there should be no appreciable effect on lifespan. There is even an argument to be made that doing so will lengthen tube life by eliminating current in-rush effect. I would leave it on stand-by unless I was leaving for vacation or other reason the pre were unattended for long periods, just as a safety thing.

I used Telefunken in mine. I found the 6DJ8/ECC88 to sound better in the SP9 MKII , and the 6922/E88CC to sound better in the MKIII. They both lasted more than five years in standby.

don't leave tubes on stand-by if you want to save your equipment and your dwelling. it's dangerous no matter how well tube electronics are designed. if you're not watching your equipment or go to bed you must turn that off.

A note about tube's being damaged by "current in-rush"...I've owned tube guitar (old low wattage stuff, high wattage English and US amps, etc.) amps that kept their preamp tubes for DECADES and were turned on and off thousands of times with and without "stand by" circuits with...here it comes...ZERO preamp tube issues. I doubt hifi amps get the abuse guitar amps suffer in their natural state (do you overdrive your preamp tubes to distortion for several hours while their in the same box as your speakers?...no you do not).