That's interesting. Jack Elliano of Electraprint fame gave me a list of trade secrets before he retired this year and he has a similar note about not running a tube amp for more than x hours however his recommendation is 6hrs so it is not unheard of and that comes from one of the best tube amp and transformer designers to have lived. His designs are licensed to several high end tube amp manufacturers.
Some interesting things in his list and some real golden nuggets in there.
The thing is heat kills electrolytic capacitors and is their main enemy. the temperature derating curve vs service life is steep and most amps do not have the higher 105degree C rating caps.