Good post and insights on ATC speakers and their high quality drivers.
However:
For example, many performance attributes depend on other attributes and you cannot have all of them- you must choose. For example, there is a clear link between efficiency and LF performance in a driver: want more efficiency? Expect less bass- and vice versa.
The takeaway with high efficiency in conjunction with LF performance (i.e.: extension) isn’t that the latter aspect can’t be had, but that it requires large size - given of course proper implementation with the right type of driver mated to its intended cab design and tuning. Hofmann’s Iron Law, as a premise, merely states that you can’t have high eff. as well as LF extension from a small size factor, but I’m guessing to many audiophiles the general deduction is being made that high eff. precludes deep bass simply because most don’t have, or won’t make room for very large size subs and the designs they reflect. In other words: large size isn’t even considered as a viable option.
Some believe ATC speakers to be on the "light" side in the bass area, whereas I find they provide for a more natural and musically integrated bass response, a trait they share with higher eff. bass systems - no doubt from a common outset of low distortion; ATC achieves this in particular by using underhung voice coils with massive motors, whereas high eff. bass systems have low distortion in the bass not least due to prodigious headroom within a given SPL range (also from designs with massive motor force, in addition to very high power handling), which is to say: less woofer excursion = lower distortion.
As Roy Gregory puts it in his review of the Göbel Audio Divin Noblesse speakers:
Those of you looking for the traditional, rib-rattling thud that many audiophiles associate with big speakers are going to be disappointed. In common with many other more efficient and dynamically responsive speaker systems, the Divin Noblesse delivers bass that’s pitch-agile, articulate and fast on its feet -- as opposed to leaden, thick and turgid. It’s not unlike live, acoustic bass. How often does an orchestra generate the sort of low frequencies that communicate on a skeletal rather than aural level? Very seldom.