i need a monitor switch with volume controls.

[birt]

A friend asked me to build a box to switch between 3 sets of studio monitors. so i figured i just needed a 4p3t rotary switch (2 poles for the balanced L and 2 for the balanced R) and that's it.
but he wants a volume control on each set of monitors. wich means i need 3 LR volume controls.

the problem is that i really don't know how to do this with balanced signals. any ideas?