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.
Aperion Verus Grand Towers & Bookshelves, Verus Forte Center, Infinity Surrounds, Ultra-X12
Oppo BDP-203, XBOX360, Xbox One
LG 65" OLED
RR2150 w/Klipsch SCR-2