Sounds like your system is very similar to mine. I have both the 4K OPPO and the HDS-42AVR. However currently the HDS is not being used because the OPPO already splits the 4K video from the audio and provides 2 HDMI outputs, one to the TV the other to the 975. As I add more 4K sources then I'll make the call about upgrading the 975 or using the HDS. It'll most likely be a funds-availability decision or MAYBE Outlaw will have something new soon (cough cough Peter? Ben?).

Honestly, once you get a 4K disc player I think you'd be all set for the near term. If you have money burning a hole in your pocket you can always get "better" stuff.