I'll have to remember this trick, to make sure I get paid!

On the other hand, I can see the need for a composer (or musician who knows what they're doing) being on hand to mix the tape when playing with live musicians – I've seen several cases where just relying on the regular sound guy on the PA has screwed things up.

I can believe Stockhausen's preparations are over-elaborate (10 mics for 2 players?), but can also believe that this excess is necessary to his art.

"Stockhausen always saw himself as at least an equal peformer when controlling the sound projection of his tapes"

This is, in part, a ruse, as the compensation tables at GEMA reward a tape piece with very little money, but if a live performer is "required" to do a mix, then the fee is several times larger, in addition to the fee the composer can demand as a performer from the presenter. While in some case, a live mix might be useful, in too many pieces the live mixer is either a mime or compensating for a poor recording mix.