Very true Tom,
If you had a SA in a studio, you could see this. I have a couple friends who do. Trust me, its both science and art.
I mean, just changing the speaker, though same specs, can cause a change in compression in live amplified music. I tired hunting it up but couldn;t find it; but a studio friend of mine had a SA chart (what we are working with here) of the same guitar, amp but a different speaker, though it was same size, same RMS, same ohm, etc. It was pretty obvious the compression difference. (FYI: An Alnico Blue against a Jensen.)