To keep it simple connect three log volume control potentiometers to an audio preamp IC output and wire the sliders of the pots to the hot pins of the output connectors. Use low resistance, say 2 kOhm potentiometers and be sure your preamp can feed 600 Ohm well. Use stereo pots for 2 channel stereo configuration.

If you need level setting switches or long cable feed (5 meters or more) ability this is too simple.

ADD: The cable length limit isn't exact, but unbalanced connections have a tendency to catch noise when cables are long and there's several devices with mains voltage supply connected together. The devices have unwanted secondary connections through their power supplies. TL072 should be able to feed those about 1Vpp or less signals to 600 Ohm. The devices cause minimal loads.

ADD2: Just noticed that you have some serious pro audio stuff. Maybe a redesign with proper balanced signals and surely full audio bandwidth amps is well worth the effort. If you can have $2000,-/pair speakers which hobby musicians can only dream of, do not enter into pauperism just now.

To keep it simple connect three log volume control potentiometers to an audio preamp IC output and wire the sliders of the pots to the hot pins of the output connectors. Use low resistance, say 2 kOhm potentiometers and be sure your preamp can feed 600 Ohm well. Use stereo pots for 2 channel stereo configuration.

If you need level setting switches or long cable feed (5 meters or more) ability this is too simple.

ADD: The cable length limit isn't exact, but unbalanced connections have a tendency to catch noise when cables are long and there's several devices with mains voltage supply connected together. The devices have unwanted secondary connections through their power supplies. TL072 should be able to feed those about 1Vpp or less signals to 600 Ohm. The devices cause minimal loads.

ADD2: Just noticed that you have some serious pro audio stuff. Maybe a redesign with proper balanced signals and surely full audio bandwidth amps is well worth the effort. If you can have $2000,-/pair speakers (and all other devices in the same league) which most hobby musicians can only dream of, do not enter into pauperism just now.

To keep it simple connect three log volume control potentiometers to an audio preamp IC output and wire the sliders of the pots to the hot pins of the output connectors. Use low resistance, say 2 kOhm potentiometers and be sure your preamp can feed 600 Ohm well. Use stereo pots for 2 channel stereo configuration.

If you need level setting switches or long cable feed (5 meters or more) ability this is too simple.

To keep it simple connect three log volume control potentiometers to an audio preamp IC output and wire the sliders of the pots to the hot pins of the output connectors. Use low resistance, say 2 kOhm potentiometers and be sure your preamp can feed 600 Ohm well. Use stereo pots for 2 channel stereo configuration.

If you need level setting switches or long cable feed (5 meters or more) ability this is too simple.

ADD: The cable length limit isn't exact, but unbalanced connections have a tendency to catch noise when cables are long and there's several devices with mains voltage supply connected together. The devices have unwanted secondary connections through their power supplies. TL072 should be able to feed those about 1Vpp or less signals to 600 Ohm. The devices cause minimal loads.

ADD2: Just noticed that you have some serious pro audio stuff. Maybe a redesign with proper balanced signals and surely full audio bandwidth amps is well worth the effort. If you can have $2000,-/pair speakers which hobby musicians can only dream of, do not enter into pauperism just now.