Octatrack - stereo inputs with thru machines

I was spending a few hours making a new track for another 'live jam' video today, with the Octatrack/Monomachine/Tempest setup.    At some point I was programming the Tempest and I was adjusting some panning, but I couldn't hear the changes.   I listened to the headphone out of the Tempest and... there it was, in stereo.   Hmm.   What could this be?

Turns out to be a simple mistake.   Thru machines on the Octatrack need to be configured correctly:

"A B"	(Note the space between A and B) Stereo pair AB.   This is usually what you want.
"A"	A only, mono.
"B"	B only, mono.
"A+B"	A and B, summed to mono!

So yeah, I had the inputs set to "A+B", which summed everything to mono!   Switching to "A B" and "C D" for the second thru machine fixed it.