Sub output: Is it the woofer size or the rated RMS

I'd add that long throw tends to rely on long coils and you get less driver control. The relationship between back EMF and inductance is a factor. Long throw designs of any size (even the TC sounds monster drivers) tend to have less control ("braking" if you will or the ability to stop on a dime and change direction).

I agree with Drew Q is very important. Think of damping like your "storm door" - a Q of around 0.3 - 0.5 will be just like you storm door closing - it is extremely controlled and you get absolutely NOTHING other than the input signal coming out (no extra oscillations and no resonances as everything is damped) Like the storm door it is very inefficient (lots of energy to get little output). If you remove the piston from the storm then it wil flap around inteh wind or oscillate freely - this would be like having a q of 1.1 or higher. A Q of of .7 is flat and a good compromise between a good response and decent SPL output but ultimately for high quality you probably want something highly dampedwith a Q lower than that and in order for this to work at reasonable SPL you meed very large drivers or the SPL is to weak.

((doubling or tripling drivers does lower their f3 point))

I meant to say does not.

The biggest factor affecting sub performance is air displacement and that can be a small cone with a large displacement or a large one with a small movement or something in the middle. Either way it’s a compromise just like all speaker design but for accuracy at any size it requires good engineering and there are plenty of good designs on the market and lots of not so great ones. Choose carefully grasshopper