As I understand it, from on line posts and a review of the GS pre and 150 Power amp,in the UK magazine, HiFi News, the changes are largely cosmetic. There may be differences in the sound, but they are not commensurate with the price difference between the Ref and GS lines. This comment only compares the GS power amp with the SE versions of the 75 and 150, not the original KT120 tubed versions.