Thanks guys. So Linux can't deal with HDR out of the "box" & yes, the mkv files are HDR. It's funny, I'm a video assist technician in film production (record & playback monkey) & I commonly work with a straight feed from Panavision Alexa's, RED Cameras etc, & it looks fantastic. I don't really see the need for HDR & Dolby vision. It often looks too unnatural to my eye. I'll eventually give JRiver MC 27 a shot, & if it doesn't help I'll continue to dual boot.
Thanks & stay safe,
G