They were designed using Crown solid state gear with the amp being a DC-300A, and George preferred classical and jazz recordings.An excerpt from an ad in 1973 :
"Julian Hirsch has already stated "Without a
doubt, the Design Acoustics D-12 is one of the
finest -sounding home -speaker systems we have
ever encountered."
What makes the D-12 sound so good? Well, as Mr. Hirsch said elsewhere "In our judgement, wide dispersion at all frequencies is the major
factor separating an excellent speaker from a
merely good one . . . unlike many other controversial
(and often arcane) speaker properties . . . the unique sound can be heard by anyone, under
almost any circumstance."
From an ad in 1975:
"Design Acoustics: our name might tell you something. From the beginning, our objectives have been to design truly different speaker
systems-systems which achieve accurate reproduction
as a result of uniquely flat power
response and wide dispersion.
Take the unconventional D-12, for example.
It houses nine high -frequency drivers producing
uniform dispersion over the entire listening area. The smooth midrange is there, as is the clean,
solid bass. Hirsch -Houck Laboratories writes:
"Without a doubt, the Design Acoustics D-12 is one of the finest -sounding home speaker systems we have ever encountered." "
Whenever I listened to them I always felt the midrange frequencies were a bit depressed compared to the rest of the frequency spectrum, which might account for you calling the sound "scooped". Mounting the midrange drivers on the surface of their panels with a routed recess area for them to sit flush would likely improve midrange dispersion over the stock configuration.
The Peerless mids used had a stiff cone surround which may have indeed become even stiffer over the last few decades. In the D-12A model George used a soft dome midrange driver on a forward facing panel (they were designed to have directional orientation unlike the original D-12). You might want to consider substituting the later Peerless mids which had a foam surround and backing cannister as used on the D-8 model. These mids were used by Infinity and Phase Linear in their Andromeda III among many other brands and models. They are not common on FleaBay but do turn up now and then.
Have fun!