Mixers

I’m using a Yamaha EMX 5000-20 for live sound and I seem to be unable to get the foldback working for some sources, even though it works for others. Am I missing something obvious? 

I’ve read that pro gear should use balanced connections, but I’ve been looking for a mixer recently and find that most have unbalanced insert points. My interface uses balanced I/O, as does most of my outboard gear. What gives?