force stereo output

I route my iMac audio through a MOTU 4pre, which has 8 output channels. I only have speakers on the stereo outs (Main L and Main R, channels 1+2). Nothing is hooked up to the other channels. Recently, when I've played some itunes movies or youtube videos, I get reduced audio or silence. I think the sound files are set to use surround sound. When I use the iMac internal speakers, the audio works normally.

I've checked Utilites/Audio MIDI setup for the MOTU, and the speakers are configured for stereo. The MOTU software doesn't have anything relevant. From what I can tell, the problem is on the mac side. Are there any other resources I could use? Has there been a change to audio output handling in recent OS X updates?

Retina 5K mid-2015

El Capitan 10.11.6