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 and pounding 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." Don Tait