That there's a "black box" aspect of synergy or a witch's brew of "X" factors going on in an amp that come together (or not) when driving a particular speaker.
To this point, I seem to recall that a while back Al may have posted something in another OP to the effect that the theory may help one identify a amp/speaker combo that MAY (??) work well and conversely MAY (??) not work well. But, and this is a big BUTT (sic), one will not know for sure until he/she tries out the particular amp/speaker combo of interest.
There really isn't a 'witch's brew' unless the designer of the amp really doesn't know what he's doing :)
To the latter question, because of the confusion that exists in the industry, you still have to try things out.
Al, it sounds to me like you got what I was trying to convey. I am wondering if there can be a better term than 'output impedance'...