Passive mixing should work just fine if the impedances and levels are the same.
Prosound = high level but high impedance ( = weak "drive") by design.
Headphone output = lower level but low impedance ( = strong drive)
I.e., the headphone output has a lower nominal level than the prosound, but it's better at forcing the junction between the two to its own nominal value.
This isn't really rocket science. All you should need to do is make a special 3.5 mm -> RCA cable for the GBP with a small resistor across the signal lines (left and right) to increase the impedance a little. I can't give you an exact recommended value but something in the range 22-100 ohms should do it. This cable will still be fully usable with only one thing connected, although the added resistor might make it more physically fragile. (Risk for cracked solder joints or wier breaks near the solder joint.)
Then there's the added complication that the impedance of the DMG's prosound will vary with the volume knob. In effect, you should always have it at maximum volume, or the GBP might still overpower it even with the added resistor.
That's all, I think. Good luck if you want to try this.
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