Does adding mass to a voice coil reduce its BL?

[tonskulus]

Yes, BL vs moving mass/cone diameter should be noted. BL of 12 is not going to give good control for 18 inch driver, but it is good for 8 inch. But still it depends on the overall design.
Take a look at isobaric systems, enclosure size can be made 1/2 to give similar response compared to single driver because of more motor force (double motor) / same radiating area.

 

[454Casull]

Many knowledgeable posts. But now let's focus the abstract discussion into meaningful conclusions. For fidelity seems to me, the core issue is control of the motion of the cone and especially transient motions.

Let's say you were buying a driver. Wouldn't part of your decision be based on motor strength (just Bl) relative to cone assembly weight (plus maybe air mass varying with diameter)?

No. What I look at is how the driver diaphragm interacts with the air to create the sound I hear. This means I'm interested in:

1) Frequency response & sensitivity, on- and off-axis
2) Harmonic distortion, but critically, it needs to be broken out into the different orders, up to at least HD5 and ideally even higher.
3) IMD (if available... hopefully)
4) CSD
5) Step response

The impedance curve can also tell me some stuff too, but I find that the info I get from it (mainly pointing out resonances within passband) is usually also in the frequency response and CSD. Well, a strongly-rising impedance curve can also imply lots of Le(i)- and/or Le(x)-related distortion but that comes out in HD and IMD as well.

Beyond that, it's just a matter of the build quality and aesthetics.