I don't know why, but every Marshall I've put an SD-1 in front just sounds better than any other amp with an SD1. The 900 series didn't have the huge bottom end that we're used to now, but they still sounded good. They were the first "good" amp I ever tried.
Peope also forget, that the JCM800s were a very hated amp at one time. That was back when we had far fewer options.