If you are diyer you can build a Kassutronic quad attenuverter / mixer.
Basicaly the first attenuverter provides +5 -5 v when nothing is patched at the input.
And there is a build option to do the same with the 3 other.
All of them are summed at the sum output.
Use center detent pots.

Erogenous Tones Levit 8 do it well too.

Cheers