I always thought that the original 16-bit Prestige and OJC CDs, including "Outward Bound" (To me, Dolphy can sound beautiful, playful, serious, adventurous, vocal, soulful, intellectual, sweet, artistic, masterful, passionate, natural, connected, deep, vibrant, ..., but never particularly sarcastic), "Thelonious Monk In Action", "Misterioso", "Saxophone Colossus", Harold Land's "The Fox", ..., even the Dolphy-Little quintet Five Spot recordings, sounded quite fine (in audiophile terms). Musically, their transcendence would be evident even through a "boom box". The K2 editions are indeed very appealing, but, with signs and rumors of increasing production, SACDs may become the ascendent truly-"hi-rez" standard format pretty soon (or maybe not). I almost (but not quite) wish that I didn't already have such a large collection of OJC and Prestige CDs, so that I could better justify the investment in a bunch of OJC K2s, but the originals usually don't sound bad at all.
I do not hesitate to purchase the latest Legacy remasterings which generally yield much more significant sonic improvements over previous Columbia editions of Miles (sorry), Monk, Mingus, etc., and usually include extra tracks.
I find the Blue Note RVG remasterings to be more of a mixed bag when you control for volume (loudness). They're inexpensive, and Rudy Van Gelder really pinned the meters for these (and, all things being equal, louder sounds "better"). Many RVGs do indeed sound better (clearer, more nuanced, more "live", less distorted) than previous editions, but some don't sound "right" (tonally unbalanced, reduced stereo effect, and/or increased distortion). Recent RVGs that offer superb sonics along with great music are Ornette Coleman's "Live At The Golden Circle" Vol.1 and Vol.2 (I prefer Vol. 1). Other RVGs that I've sprung for and enjoy include Wayne Shorter's "Speak No Evil", Grant Green's "Idle Moments", and, recently, Bobby Hutcherson's "Dialogue".
I've been playing a new CD reissue of a record that came out around 20 years ago, Sam Rivers's "Waves". It's an invigorating, varied and creative program of music - Sam Rivers on saxophones, flute, or piano; Dave Holland is central with his magnificent bass playing; Joe Daley plays some of the hippest and most flexible tuba ever; and Thurman Barker makes his usual fascinating and sensitive contributions with drums and marimba. Getting "Waves" fortunately inspired me to finally purchase the excellent Mosaic Sam Rivers Blue Note box set (and just in time); it arrived at my house last Wednesday, which was the day that Mosaic ran out of it
Finally, I've noticed that most of the new jazz releases that I've purchased on CD over the last 3 to 5 years have generally improved sound, compared to CDs I purchased before. Even some pop records are being released with excellent sound...