Thanks for the info. I'll make sure that the amp is unplugged when I change the power tubes.
The only reason it is suggested to turn off the amp(not just in standby) is to allow the rather hot envelopes to cool enough for you to handle the tubes. It is best to use some type of insulation to pull them anyway because skin oil left on a tube, if the tube is run really hot, lead to the glass cracking. The hotter the tube envelope to more of a risk.
Negative bias comes up first so you can hot swap tubes with no ill-effects, even with the amp out of standby. Receiving tubes and small power tubes have a controlled time delay for heating the filaments to full emission, 7 seconds. That means the tube is not conducting immediately, but gradually increases conduction over that time. After 7-10 second, the amp should be fully ready to apply full signal through it.
Were the old pre-amp tubes defective? I noticed you put in a new "Tung-Sol" tube. Unfortunately the name was sold to New Sensor who stamps the highly regarded Tung-Sol brand name on any sort of tubes they can get(they also acquired the name of "Mullard", and "Svetlana" with no connection to their former quality either, essentially the same as Yugo acquiring the name Ferrari and stamping it on their cars). Do not be confused but the slight of hand, real Tung-Sol tubes are in very short supply, usually in a boarded up old TV repair shop or industrial parts department of a plant somewhere.
Small tubes, called Receiving Tubes, such as the 12AX7 , were designed to last many years unless physically damaged. Power tubes do consume themselves over time but small receiving tubes might need replacing every 10-20 years in a head and 7-15 years in a combo where they are subjected to more vibration abuse. I have some test instruments, HP RF precision signal generator built in 1968 that still has all original tubes and maintains calibration, it is on for most of the day every day since it was new. My old McIntosh tube preamp built before that still has all original tubes and still meets spec in all parameters.