Yeah the soundperson should be a musician, or at least a very perceptive listener. When my partner and I started a recording project recently, we were lucky enough to find a first rate home studio run by a very fine acoustic musician. Knowing what the instruments and the voices are supposed to sound like is 90% of the battle. Also keep in mind that mixing for a particular room can be very difficult. If one wall is all mirrors and the others are divided into high-backed booths, there's no way to get the sound just right for every spot in the room. On stage we mix our own so there's nobody else to blame. Another good tip: spend as much as you can on good microphones. Makes a ton of difference, and sounds better than any pickup.

Chet