I read briefly through the whitepaper and I get your point, but I disagree that a good voltage source amplifier is large and expensive. And with class G/H designs, efficiency is pretty good nowadays. Class A is the pig in the room as far as efficiency, and for that, high power output is going to be costly in terms of cooling and power consumption.
It's been known for over 50 years that speakers designed during the tube era sound 'dead' on SS amps. It's as you say, because the tube amp will increase its voltage output at the resonance of the speaker and maintain almost constant power to the load that is varying in impedance across the frequency band.
Modern speakers are designed to work with modern amplifiers though. I was an all tube guy for many years. I held out until the late 1970s with my home brew tube monoblocks. Nothing sounded fatter, played as loud, or seemed as tolerant of overdrive as my tube amps. My first SS amp was a Fisher TX200. It crackled when it ran out of gas. Years later, someone loaned me a 40wpc Dynaco SS amp. Same thing. Seemed to lack bottom end. In the late 1970s, I picked up a Marantz 2270. For sure, this should have more bass. It didn't. It sounded grainy, thin and brittle and when driven hard, it would oscillate and that would manifest as audible clicks or chirps in the tweeters.
A year later, I brute-forced it and got a PL D-500. That broke the barrier, but it wasn't perfect. It wasn't until 1984, when the MOSFET amplifiers came along, and would finally replicate the clip tolerance of my tube amps, while offering much wider bandwidth and power.
Now those Lowther speakers are interesting, but they come with a great many (and ridiculous) compromises. The size of the horns, just to get down to 50Hz, and the problems of intermod using a single driver across the whole band. Well, they're probably okay if playing string quartet music, but pipe organ, or marching band music? I don't think so.
My experience with seeking out amplifiers for reproducing low frequencies has taught me that for power efficiency and woofer control with exceedingly low distortion, a transistor amp is the way to go. Nowadays, I run industrial amplifiers which can work all day into 1Ω loads without strain and damping factors in excess of 2000. And I do get Nelson's point about overdamping. I did lose some bass by going to the much higher gauge wire. But the bass I have now is more accurate without the boom.
If I were to try to recreate my present system with vacuum tubes, I would need 15 tons of air conditioning capacity and a direct feed from Niagara Falls. I simply could not even afford to turn on the system for one hour.
The present system is clean, quiet, cool-running and relatively maintenance-free. I could never see myself going back to vacuum tubes.