DAC nuances

Flagship DAC's of old tend to age well. IMHO, DAC designers of old focused on actually listening to their gear to ensure a non-fatiguing sound that was well-rounded and pleasant.

newer isn’t always better

Ditto. The ASR crowd constantly pushes the narrative that quality can be boiled down to a single SINAD number, but often this result is completely divorced from what actually sounds natural and enjoyable to us (meaning the typical audiophile here on this forum). Doesn't surprise me that your "old" K03 still impresses! Very nice player; I almost bought one. 

Every 20 years (or so) I get fooled into try the latest vaunted RME DAC product; 5 seconds into actual listening, I can't get out out of my system fast enough! First it was the Digi PAD sound card, and more recently their ADI-2 Pro DAC. Great SINAD number, though. Ugh, never again.

By reputation, your Esoteric tilts warm.  So one modern DAC brand that leans in that direction is GoldNote.  My DS-10. which I love, uses the AK4493 chip, which has a reputation for warm, more “natural” presentation.  In some applications, Gold Note uses Burr Brown/TI chips, which also have that rep. 

Btw, generally agree with @mulveling , but the AK4399 chip in your old cd player had a 123 db snr, and in multiple parallel mode, you could add a couple db to that.  Do that was absolutely state of the art at the time, and nothing to sneeze at today! 

Thanks for taking the time to share your thoughts and experiences. This morning an article from Headphonesy, A Compilation of 50+ Blind Tests Reveal the Only Gear Upgrades That Actually Matter was in my feed. There is section DACs included as one study. The Esoteric, Technics and Eversolo DACs all sound "good", it was the nuances between them that I found interesting. 

A good Dac is a good Dac regardless of age. Happily listening to a circa 1995 Audio Note Dac-3 Signature and it sounds fantastic.