To be more specific, my tube amp has a 1040 joule power supply. In my "un-technical" way, I interpret that as "head room" because my amp presumably can handle short term power demands that exceed its rated power output of 150 wpc, subject to FR, speaker impedance and tap output impedance. Am I mixing and matching terms and concepts here??
Yes.
Its important to understand that the concept of 'dynamic headroom' and the resulting spec was entirely 100% marketing on the part of the manufacturers of inexpensive amps and recievers during the 1970s.
A worse amplifier has higher dynamic range? Worse how?
I did explain that in my prior post, here it is again: the amp has a small power supply that will not allow it to operate at full power continuously. On top of that, it would probably overheat if it did due to poor heatsinks. Further, it is biased class AB and likely exiting the A region with less than 1/2 watt output. These properties will allow the amp to put out brief spikes of power that otherwise its design does not allow in a steady state condition. The higher the headroom number (in db) the cheaper the amp.
Class D FWIW is not an exception to this rule of thumb- the best class D amps will make about the same power whether continuously or not.