As the components and parts that support the given A to D converter chip are upgraded by a knowledgeable person, with good powers of discernment, the missing differences come back on line and are noticeable.
Korg is not there to make you a reference grade piece.
A knowledgeable audio tech with aspirations in high end and the will to do so, can get int your Korg, analyze it, and then correct the circuit to bring those missed differences back on line.
The Korg is not a perfect black box, it is a faulted complex device, like all of them, from everyone.
Right now, one good guess in the right direction, is that the seldom found Ayre A to D converter is likely to be among the very best.
https://www.analogplanet.com/content/analog-digital-converter-shootout-which-sounds-best
Korg is not there to make you a reference grade piece.
A knowledgeable audio tech with aspirations in high end and the will to do so, can get int your Korg, analyze it, and then correct the circuit to bring those missed differences back on line.
The Korg is not a perfect black box, it is a faulted complex device, like all of them, from everyone.
Right now, one good guess in the right direction, is that the seldom found Ayre A to D converter is likely to be among the very best.
https://www.analogplanet.com/content/analog-digital-converter-shootout-which-sounds-best