With any amplifier you have to know what the amplifier's abilities/limits are, at what level it operates most effectively, as it's supposed to. You start running into problems (typically too much heat) when you over do it. That also means that you have to know what particular mode that's to be used and how it 'works'. [The reason for that 4:1 ratio of modulated power applied to carrier power is that 100% modulation is produced. Any more than that always results in distortion. A modulation of something like 85% is probably all that should be 'aimed' for, gives you some lee-way for sound 'spikes' still being under 100%, and the typically 'ear' will never hear any difference. This is one of those things where "bigger is better" just does not apply.]
The important thing is that the signal being fed to the amplifier doesn't exceed the amplifier's ability to handle it well. Back to knowing what the particular amplifying devices (tubes/transistors)can do well, and when to quit so that they are not over stressed. There are limits, and those limits are NOT what is commonly thought to be (Unless you just like to burn things up). It all depends on time too. As in how long is that amplifying device actually running/amplifying. Some modes are of a higher 'duration' than others. 'AM' mode is a 100% duty cycle mode, an amplifying device is running 100% of the time. 'SSB' mode is typically a 50% duty cycle mode, when there ain't no sounds (between syllables/words), those amplifying devices are turned off. The devices work less with SSB than with AM. Since one of those 'limits' is how hard they work, they can work at 50% for longer periods than 100% and not get 'over-worked'. Make sense? So, just by going with the duty cycle thingy, half power on AM rather than full power on SSB is how to do it, and all that you should expect.
Then why do 'they' say it can do more than that on AM? Two reasons. First, 'they' don't know what they are talking about. And second, 'they' want to sell something, and/or are 'bragging'. 'Normal' advertising.
Next step. Pay attention to what the manufacturer's of those amplifying devices say they can reasonably/typically do, NOT what they can be 'pushed' into doing for a much less length of time. But it's your wallet that gets 'hit' so treat it like you want. A typical 2SC2879 will 'do' something like 100 watts with a SSB signal. With AM, about half of that. Usually, something around 1-3 watts will make a 2SC2879 do everything it can do in a reasonable manner. Four times the number of '2879s means something like twice that amount of input power. Just the way it works.
All of the above is a very general statement. That means that I am not familiar with your particular amplifier and how it is designed/built. The best advice is to start 'small' and work your way up higher in a very conservative manner. Not all that hard to do if the output of your radio feeding the amplifier is variable. If it isn't easily variable, then it gets a little harder. Same idea, just harder to do. (You can make a Sear's lawn mower into a roaring demon, but it ain't cheap and it ain't easy, and why would you want to to start with? Get a bigger lawn mower.)
The next best thingy is to find someone who really does know what they are doing, and let them 'do' it. If it sounds too good to be true, you can bet that it is. Cut the 'claims' in half and you're a lot closer to fact.
Good luck!
- 'Doc
...runna pile-up on 6m SSB