An interesting comment from a very knowledgeable source, on a different thread, prompts this next question.
The was along the lines of 'a 250watt rms speaker with an xmax of 4mm is not going to be any louder than a 150watt rms speaker with the same xmax. This is logical as a 10" cone moving 4mm is going to shift the same amount of air as a different 10" cone moving the same distance.
Does that make the xmax a more important comparison that mere rms handling? Doubtless this will set off a long thread about characteristics etc but the previous question was a brilliant thread and the knowledge gained was significant. I am hoping the experts can indulge me once more.
The question on what happens (electronically) when I turn up the treble control can wait for now.