Everything !! First of all, I am using a pair of Proac Studio 250 speakers. They aren't the most efficient in the world. 89db I believe. The M120's have gobs of power compared to the 88. Every note seems presented effortlessly as compared to the 88. The 88 ( after looking back at the sound as compared to the 120's) seemed strained and maybe a little congested compared to the 120's. I felt happy with the 88 for a little while. Maybey 7 or 8 months. But then I grew tired of the presentation. I am not the most electricaly technical person here so I can't explain the theory of operation but, there are 4 KT88 output tubes per channel as opposed to 2 with the 88. That has to count for something right? Not only that but, the 120's are great heat source for a cold room :-)
I felt that I couldn't go wrong. I traded my Magnum 88 and $900 for the pair of 120's and I haven't been happier.