UR44 'Output knob'

Hello there,

I recently purchased a 2nd hand UR44 (the old one, not the ‘C’ version) because I want to experiment with a quadrophonic (4.0) setup and mess around with some surround stuff for an upcoming project.
Everything set up quite easily but there’s one big thing that I keep knocking my head against which is the ‘output’ knob on the UR44.
I want to set the volumes of my speakers manually (either trough hardware or in the sofware mixer that came with the UR44; dspMixFxUR44) and then have one big knob to change the volume of the whole 4.0 mix.
The output button on the UR44 however just changes the volume of the main out mix while the other ones stay at volume (as far as I’ve found there is no way trough this piece of hardware to set the other output volumes).
Is there something I’m missing? (I read the manual twice now) or is there a certain workaround that I can use (or was expected to use to begin with)?

Thank you very much in advance!