I'm a recent convert to the world of IEMs. My set up, which is relatively simple, is a headphone amp run to the aux out of our mixer, then UE900s plugged into the headphone amp.
It's been brilliant, and i'd recommend anyone who is thinking about using this set up, to give it a go!
I can't believe i hadn't done it sooner.
Im sure a ambient mic would be good, but i don't think it's something you need to invest in straight away