First time I've ever seen the UM18-22's referred to as "Low power handling" but you live and learn.
I'm not going to disagree with you (well ok I am a bit!
) but care to explain as I don't understand.
What does "..may be ruler flat but you won't get the same midbass feel as a high sensitivity, high power handling pro driver" actually mean? What is "feel"? What you "feel" is what you either hear which are sound waves exciting your ear or "feel" as the physical sound waves resonate or vibrate your body in some way. That can be measured, trivially and as far as I'm aware is a combination of SPL, frequency and amplitude. Of course it all works together and I get that if you're feeding low sensitivity speakers with an underpowered amp you're going to suffer lacklustre results. But from a pure "feel" perspective the more SPL the more you'll "feel" it but that's common sense too. Are you suggesting that there is something else that can't be measured at play? If so what is it?
Ultimately there is supposed to be a 1 to 1 relationship between the input and the output. Input X and you get Y. In a perfect system X = Y in the sense that if the input signal is set to generate 85dB at 20Hz your output will produce a 20Hz sound at 85dB at the MLP. That is exactly what you're measuring (and aiming for) with REW. REW produces a sweep and then measures the in room response to that sweep and to be honest that's all that matters as that's what we actually hear. The goal being that across as much of the range as you can you try and get that exact 1 to 1 response so that the output you hear is exactly what your input intends. Ultimately you don't get that only from the speakers as room resonance and room response greatly affects what you hear. You are also at the mercy of psychoacoustic and biological effects (humans perceive low frequencies as quieter than they actually are so you apply a house curve to boost them) so this may drive both placement and any EQ you apply to artificially change the speakers response at any one desired range. Objectively though, concepts such as "slam" are measurable unless you can tell me why not.
Now accuracy is an interesting one. Explain the physical theory to me because you talk of magnets makes little sense to me as far as modern speaker design and material goes. The strength or size of magnet is not the deciding factor as to the quality or ability of a speaker. Magnet strength is one of a number of different properties which dictate the quality of a speaker and it's usually not the most important one. Indeed too strong a magnet can actually cause problems in subwoofer speakers and there is absolutely no rule that says bigger/stronger is better. If it was then everyone would have maxed that out a long time ago. It's a very complex area and almost everything in speaker (and cabinet) design is a compromise so you simply can't make a blanket statement that stronger = better. That's not to say those speakers are not superb but it having a stronger magnet likely means they've had to make compromises elsewhere.
But as I said, I'm an armchair pundit not a speaker designer so I'm sure I'm grossly simplifying it all to the point where I'm talking cobblers. Happy to learn though.
G