. . the sound improved according to Barry"
. . but not hugely so. You notice it mostly in the ones that didn't sound all that great to begin with, such as the Vienna "DLvdE" and the Chicago M9.
You can get better performances of all the works by Mahler, but Boulez's cycle is notable for its generosity (in terms of what's included), consistency, great vocalists (except for the two in "DLvdE") and the fabulous playing of Vienna, Chicago and Cleveland. I like my box very much.