I'm currently unable to quote other members' posts, so I have to summarize
a couple of things, going back a while in this topic. Sorry I can't quote.
DynaGroove and DynaFlex were indeed separate, as several members have
pointed out. The former was the EQ system introduced around 1963/4, the latter the
thin RCA pressings of about 1970 onward. I only know DynaFlex pressings of
classical LPs, but especially when they were compared with earlier pressings on
heavier material, the sound was ghastly: thin, shrill, washed-out.
I liked Aaron's story about Richard Kaye's on-air comments about Ravel's
Bolero on one of the initial DynaGroove releases around 1964 because I own the
record. Munch/BSO (a remake for them). It's incredible. The ultra-quiet
beginning is at the same dynamic level as the end, with the full orchestra blaring
away. "Loud" indeed. Koussevitzky's 1947 BSO recording has a wider dynamic
range, and so do the Victor LPs of Munch's 1956 BSO version. Around 1964 a
friend described DynaGroove as "acoustical stereo."
A few of the things that were "DynaGrooved" for LP did appear in RCA's
"High Performance" CD series a few years ago, and I've heard that they sound
superb. The introduced distortion was evidently removed.
As a late friend used to say (paraphrasing David Letterman), DynaGroove was
"another stupid RCA trick."