I think the solution is simple.
If you can only afford 1 sub, then go for the biggest/beefiest driver you can afford. Such as the RF-19 or SI-24 or the IST-32.
If you can afford 2 subs, then go for the two biggest/beefiest drivers you can afford.
See the pattern yet?
You can see the distortion profiles of a number of various woofers on http://data-bass.com/systems
It becomes clear that a LMS-18 or RF-19 or SI-24 well out-does a B&W 8 or 10 at nearly any frequency or SPL level.
They extend lower while also having less distortion; and this is VERY audible.
If two subwoofers were EQ'ed to within a 1db of each other and kept below 5% distortion at all times, that's where things might get difficult to decide. The cheaper of the two having the better value-ratio of course!
In order to play a wide bandwidth like 120hz to DC, the driver with the highest xmax and overall power-handling has a MAJOR advantage over the other (that's assuming the sensitivity-rating, motor-force, moving-mass, box-size or any combination there-of, isn't throwing a curve ball at you...)
If you are 2-inches below xmech and just coasting along like a RE-18, the driver is gonna sound pretty clean.
But if you are a Dayton HO-12 trying to be the little engine that can't, well it's gonna sound awful when pushed to anywhere close to the same levels. (Or a SI HT-18 that only does 600w and a 1/4 the xmech.)
It's just like a car, they don't like running near the red-line vs having 100's of horse to spare.
Think: Smarty vs Lambo going up a hill. Pedal to the metal vs still in 1st gear.