cheers! your assumptions are accurate.
I'm pretty sure I have Reaper configured to have the Output Range to 4. At least that's what it's showing in the Preferences under Audio - Device.
Yes, track 3 is getting routed to 1 and 2. I think this is my issue. I've unchecked Master Send on all three of my tracks and routed them using Hardware Outputs. 1 and 2 are working the way I envision but 3 gets routed to 1 and 2. Under "Add Sends", there are only 2 listed and also Parent Channels showing are 1-2 or 1, 2. No 3-4 anywhere there...
Hope that narrows it down. Maybe there is a better way to do this?
Thanks for the thoughtful responses.