Thanks, sometimes videos do not work for me (work blocks them, for instance). Here are my comments just to throw them out. This is not something I claim a lot of expertise upon, pretty far afield from my career path.
BL = magnetic flux times path length in a voice coil? Not sure I completely agree with (1) and (4) since there are other ways to skin the cat but as a general rule I could not disagree. This is not my area of expertise by any means. For decades my subs of choice have included servo control, starting with my own design and now commercial products from Rythmik. Having a good driver and supplementing with servo works best for me to provide good (high) output and low distortion. Only so much you can do starting with a lousy driver; the phrase "polishing a turd" comes to mind.
I think (2) is a better, more concise way of my statements about integrating the music (more or less).
On (3), when I had one sub, I found placement between the two L/R speakers worked best for me, partly due to a fairly high crossover and modest slope. This was in a two-channel music system, long before Audyssey and such. Later I designed a higher-order crossover and was able to set it lower and placement became less an issue. My current system utilizes four subs in a mono swarm of sorts, with the front two operating as a single pair and rear another pair, phase/time aligned as two pairs, then I let Dirac Live correct the entire quartet once I had the two pairs in synch.
My 0.000001 cent (microcent) - Don