To be perfectly clear, energy stored in the supply is how an amp can handle an orchestral crescendo that would cause it to exceed its continuous power rating, correct?No. The extra energy is useful to reduce IMD at higher power levels. Essentially it allows the power supply to have less noise- and less noise in the power supply results in the lower IMD of the amp.
Since this is a conversation about having enough power, what about going for a larger tube amp like 100 wpc.As you increase the power of most tube amps, the output transformer has to be able to handle that power and at the same time make bandwidth. The problem is this basically doesn't happen; with output transformers as you build progressively larger devices, bandwidth is increasingly more difficult to obtain. For decades, 60 watts was the 'sweet spot' for push-pull tube amps, where they could make the power and still have the bandwidth needed (5-100KHz). You may not think that bandwidth above 20KHz is important, but unless you have a great degree of feedback (above 35dB or so) that bandwidth is important to prevent phase shift which can cause issues with tonality and the soundstage presentation.
Another thing to think about is the simple fact that 100 watts is not a great deal of power over 60 watts insofar as the ear is concerned- its slightly less than 3dB which is barely louder to the ear. But a 100 watt amp might have other properties (assuming bandwidth isn't an issue) at lower power levels that could be in its favor. So its a mixed bag.
OTLs get around the bandwidth issue by getting rid of the output transformer, so that tradeoff does not exist in them when going to higher power levels- a 200 watt OTL can be as fast, dynamic and transparent as a smaller one.