You can go a little crazy searching for the best possible sound for the recordings. I have the RCA Box but because I hear some distortion on Jack the Bear and Koko, I've gried to find other transfers. I now discover that I have four other sets that have much of the best sides from this era. My favorite is from France, the Dreyfus cd entitled "Koko". Dreyfus did an additional cd called "Take the A Train" which has 21 cuts from 1938 to 1950.
I also have a 2cd set from Avid, "Highlights of the Great 1940-1942 Band". They claim to use "Advanced Transfer Technology" but add a riders saying the quality of the sound depends on which studio (Chicago is best, LA worst) was used and you need to compare it to other reissues to understand why they advertise "remarkable sound".
Another Avid cd "Jack the Bear" containing recordings from '27-'45 claims to be in "3 Dimensional Sound" which produces a stereo effect".
Finally there's Robert Parker's "Jazz Classics in Surround Sound. Duke Ellington the Great Bands of the 1940s".
I haven't listened to these in years but I hope to do some comparisons this week and will report back . (I'm not really an audiophile and have 81 year old ears. )