Sounds like a fair question for Klaus / Alex. I look forward to reading more of this thread
I will have to stand with being safe and protect the equipment and your investment of course.
It has been said may times before by a great number of manufacturers and I include myself (so does Klaus for that matter)...when making any changes (hardware) to audio equipment, turn off the amp (s) first then the preamp and follow with the remaining components...the turn on process is the inverse, sources first and amp (s) last.
This apply of course to the initial setup and connecting everything together, and the corresponding swapping of cables that we all go through! Now, if you are going to swap tubes...PLEASE turn everything off for the safety of the equipment and yourself! When you change tubes, you will still have a period of warm-up as the tubes settle in and most important they stabilize within the circuit.
We have a new member, Kalel....welcome to the circles

Regards,
Alex