Be able to mix many voices. For some we'll use 1-3 vocalists, and others 0-11! So, be prepared for a range. Also, it's helpful if you can shorten off-vocals and tune/master vocals and use voice effects/enhancements. Being able to time and adjust voices is a necessity. I can mix for sub-unit covers (up to 3 voices) but no more than that. I'm looking for multiple mixers.