Wow, do you have a bunch of things wrong:
• Six 8" woofers give the response of one 8" woofer, but with 6 times the excursion, 6 times the power handling, 6 times the heat dissipation and 1/6 the distortion and non-linearities one 8" woofer doing the same work. These are approximations, but they work out to be fairly true.
• The cumulative weight of the voice coils, formers cones and air load is a non factor... they act independently for each driver, not as a summed group
• 8" subs tend to have a moving mass including air load around 3 oz. That is substantially less than this sub so if you are trying to tell me that six, well-designed 8" subs will be as slow as the best 18" driver, sorry, you are wrong
• Once you start getting upwards of 18 inches for a sub driver, the size of the magnet being able to control the driver has already started diminishing. That's just a fact. Sorry.
• Moving mass is never advantageous when you are designing a loudspeaker driver. It is a necessary evil that can be utilized when engineering a real world design, but it is not ideal.
• Six, well designed, 8" sub drivers will eviscerate any 18" sub driver any day of the week
• The lowest note on most instruments is a low B which is about 31Hz. You can go even lower, a super low E is 20.5Hz, but nothing but a pipe organ or synth can hit that (OK, John Paul Jones' crazy bass string things can do that, but, hey, those are not normal as is he... in the greatest way!)
Bottom line is this:
• A phalanx of small subs is better than one huge sub any day
• The reason there aren't more subs like Paradigm's is the expense, plain and simple