NES compositions frequently use the third channel (triangle waves) to make a baseline. However, it usually doesn't sound authentic unless you lo-fi it up a bit. The trick is in the Oscillator Editor; after selecting Base Function of Sawtooth, select the wave shaping function of Qnts. I usually use the parameter of -14 or so.
I also like to change the filter parameters to make it filter less, and turn up the volume some so that it's easier to hear.