^If I may; it's not a typical eq. It's really a bass only booster.
By compensating for the natural bass roll off of the 10' woofers in a relatively modest sized sealed box cabinet Jim Thiel (and others including B&W, KEF ,etc.) was able to make a truly full range speaker with a high WAF.
The disadvantage of this approach is a bit more cabling in the loop, and more demands on the woofers and amplifier (more on this later), and of course the rightly or wrongly audiophile thought of anathema of putting more electronics in the path.
The advantage is of course true full range bass (!) in a much less expensive to make (well) and easier to sell smaller cabinet. Some have suggested less energy storage, and tighter response from the smaller woofers than would otherwise be necessary as a plus.
As for the extra amplifier demands, yes it's true, but in the case of the Theil 3.5's it's not quite as challenging as it might first appear. The 3.5's are spec'd as 4 Ohm nominal, 4 Ohm minimum, and they do have a very smooth steady impedance, but at 70 Hz, the eq gradually starts to put more and more demands on the amplifier to compensate for that typical sealed box roll off, but just as that happens the impedance plot starts to increase and ergo so does the sensitivity. See, quite brilliant.
Jim put one small switch on the eq to access bass response to either 40 Hz or 20 Hz to adapt to different sized rooms or musical demands. By musical demands I mean bass volume. Some music such a much popular music doesn't have much below 40 Hz but is typically played very loud in that region, where as other music such as much classical music might go deeper but not necessarily as loud there. The 3.5's can play deep bass quite loudly in typical rooms, but truth be told, one can only bend the laws of physics so far; very loud, very deep bass still needs a much bigger and much more expensive speaker system.
As Jim Thiel once told me, unfortunately the eq raised a red flag to many audiophiles with preconceived prejudices. One might imagine that with today's digital room correction eq's this strategy might return, as one could not only dial in accurate bass to anechoic standards, but also to customers individual rooms in the very frequency range most likely to be problematic.