my only solution involves specific sound card (many but not all soundblaster live/audigy/audigy 2) with a third party driver (kx project) and a kxdsp plugin I wrote
I can get good control for up to 4 audio sources and up to 4 DJ's (each with own headphone output) using just one audigy 2 (for more than one DJ, you need a little inexpensive external circuitry, i.e. a mic preamp for each DJ) plus VOIP included in the mix
even a soundblaster live handles 2 DJ's with no problem
the mixing is pretty useful, with each DJ able to control how much of himself he hears (I like to hear myself, others don't), how high the music is in his headphones, can listen to a PFL channel, can monitor what's going to the "encoder" (i.e. to air), what's going to the speakers
anyway, if this sounds like something that may help you, let me know
11th commandment - don't be a dick.