"During the middle of the set the gain went from 12 to 16. "
Hmm. I find it difficult to understand - could you help me (non-native english speaker) understand what happened.
Obvious things to look for is that the cables are correct type (guess you have done that). The gain on line inputs on the 722 could be a little confusing -- I´ve stumbled on those before so maybe one check extra.
The system has plenty of gain left though above the 40 ov the sax, guess that is not your complaint: you could switch the sax to high, you could switch the 722 to mic input instead of line.
Gunnar