@DVDDoug , out of interest can you play back mono files correctly in Win10 on anything non-ASIO? I tried switching Reaper to use DirectSound or Waveout and it also gives this doubling/stereo effect on a mono effect. Switch back to ASIO and it is correct mono.
I am wondering if this is actually considered a feature in Win10
I have previously used Win7 for most of my audio work and it doesn't have this issue. Also, I did check my laptop soundcard settings and it doesn't even support Spatial Audio so it can't be that.
Interestingly Win10 has a specific checkbox to make everything mono, so I am wondering if it is the OS doing this stereoising by default. Even if I turn on the mono switch in Win10 it still sounds strange, like it is mono doubled.
more info here...
https://www.thewindowsclub.com/mono-audio-windows-10