I found the Soundstage similar to the 99's in width and a little deeper with more precise imaging with standard cable... On balanced cable for 99's I like the Soundstage much better but haven't tested balanced with the Hd6xx yet
The 99's seem to respond much better to eq than the HD's without that I might like the sound of the HD's better... To me without eq, the 99s bass bleeds into low midrange and messes with male vocals especially... But with a little eq, I love them.
Excited to learn more about the upcoming 105s!