If I understand your question correctly, you are planning to hook up two small speakers as L/R stereo mains, and a subwoofer to handle the bass.

Given that assumption, you would simply wire the sat's to the 1050's front L/R speaker terminals and run a single RCA cable from the 1050's sub/LFE output to your sub's lowpass input(RCA). You than select one of the 1050's (6)possible crossover settings(60,80,100,120,150 or 200hz)which directs the highs to the speakers and the lows to the sub. There is no "downmixing" involved since you will not be using any DSP modes such as "Stadium" or "Jazz", just plain old "Stereo" mode. You need to tell the 1050 that you have "Small" speakers connected so the bass management will do it's job.

Hope that helps!