If you can find a used Yamaha DSR70Pro, that would do the trick nicely. It was originally intended to be an add-on to Yamaha's old DSP1 processor to upgrade it to the 'new' Pro-Logic. It has six channel in and six channel out with master volume control (no remote though) and has on board Pro-Logic decoding too (which you would not need).
I have one I'm not using and would sell if you are near the Ottawa area.