There is no substitute for cubic inch or in this case surface area

Not totally necessary if you have good design vented speakers with dual woofers 

you can get very good deep bass $$ is the limiting factor.

great stand mounts ,with a pair of good subs click all the boxes 

your $$ budget makes or breaks the speakers design,build quality 

from my experiences.

I've often thought of woofer speed not as acceleration but deceleration. The ability to stop fast is also speed and it affects the following tones since if it hasn't stopped it partly obscures the next tones and it reduces the dynamics since the next tones start from a higher noise floor. Basically it's the Q of the bass design that describes woofer control with 0.5 Q being maximal damping and 0.707 Q being flattest response. Above 0,707 the bass becomes more and more bloated. I might add that below 0.5 the bass becomes thinner and rolls off too quickly. There's more to balancing Q and bass cut off but that's a longer tale.

I'm always going to bat for large, stiff woofers with high excursion capabilities over small woofers no matter how many.  The distortion specifications are pretty clear.

The good reasons for small woofers are usually more related to having limited bass not waking the room modes that are deeper.  So much of what is attributed to using small speakers because they are faster ends up being caused by room mode issues, not speed. 

As for soundstage, not sure but maybe it has more to do with wide baffle speakers?

Interesting. I would have thought soundstage was more related to the mid and higher frequencies, since our localization of low frequencies is very poor. If the 12" drivers were operating up into the midrange, their higher directivity at those frequencies could result in less room interaction, and a higher direct to reverberant ratio, which could be more effective at preserving directional information. Just a thought.

Interesting. I would have thought soundstage was more related to the mid and higher frequencies

I know a lot of people have reported back that by adding the a good sub, the soundstage also was improved so I think low frequencies are related to soundstage.