After scouring some threads I think I’ve found the stereo issue.
Roku can accept AAC5.1, but instead of passing it along, it decodes it to pcm stereo.
Since roku is telling Plex it accepts AAC, Plex is transcoding to that, rather than ac3, which would be played back in 5.1 as expected.
I’m now trying to find a way to force Plex to serve ac3 to roku, but no luck so far.
Suffice to say I’m pretty disappointed with roku so far.