As someone who also loves the Roksan sound and who upgraded from K3 to the Caspian M2 I'm listening to right now, you already answered your own question in your original post.
The M2 puts a finer point on detail than the K3 without sacrificing bottom end, in fact the way it reaches down and keeps hold of the bass is compelling. Impeccable timing that requires only well-matched floorstanders to sound powerful rather than secondary amplification (sub) smearing the timing.
There are very good reasons this 2010 amp took so long to come down in price and it doesn't take long to find them. You can keep your class D, room correction and on-board digital.
Well-proven traditional class AB running in true stereo without digital accoutrements hasn't suddenly found itself bested by the technological advances, marketing campaigns and internet forum chit-chat of the last decade and a half believe me.
The latter 3 paragraphs are exactly my findings over the yrs.
Although going for a modern class-G amp in my main set that melds the two a&b classes in a more efficient way yet staying true to both classes.
I still, as I over state 😊 regularly go to a 33yr old class a/b amp and another coincidentally a k3 to have it musically played out to me that we haven't gone further forward with better gear much at all in recent yrs.
Only connectivity has advanced mostly for the worst from inferior types of connections via Bluetooth to lossy audio from the most popular sites like Spotify.
We can all qoute as many acronyms as there is (there will probably be another by the time I've rushed this) but the loin share of streaming is inferior to the humble CD and as I've found even when stated it's better there's still a difference for the better to be heard when switching from tidal to say the Marantz CD 60.
None of this of course is an issue for most as hifi isn't a thing to them and the convienience of streaming for the masses is why it's overtaking everything.
I'm off to the maverick hifi show in Barnsley for the weekend which has plenty of rooms for me to further my opinions of course.
I certainly want to demo the heaven Billie amp at some point which from reviews seems like it may change my mind about class d.
Being open minded and too analytical for my own good at time's means I can and only will speak on what I've actually heard for myself