It has been my experience that of everything is workig right, and the output circuit is efficiently designed, the maximum reliable power output from a linear amp running standard AM is about equal to half the total rated plate dissipation of the tube(s). i.e. for a pair of 6146's, 20-25W carrir, 80-100W PEP.
An amp with four 811A would be ok for about 100W out under carrier conditions. Such operation runs the tubes at rated dissipation during carrier conditions, with the attendant high level of wear on the tubes. This is just another way of stating what has been said re: efficiency, but it's an easy rule of thumb. For my NCL-2000, which has 800 watts dissipation, I could run a 200W carrier. I dont. I run a 100W carrier, and the amp stays cool and reliable.
