[p750mmx]

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My question is should I use hardware mode or interlaced mode ?

I saw the receiver has HDMI 2.0 and HDCP 2.2 support, but missing the "a" specification on HDMI? In that case, HDR isn't a option, you're correct.

In relation to displaying 3D. If the motherboard has HDMI 1.4 only, then you could go for a 1080p/60Hz resolution and the "hardware" choice or for a 4k/30Hz solution and choose Interlace. I would prefer the 1080p option with HDMI 1.4 only as a available connection.

Interesting question:
What is the difference between Hardware mode and Interlaced mode?

I don't have a hardware for this so I can't test it.

Hardware mode will reproduce true 3D Blu-Ray MVC "Frame Packing" 2 frames of 1080p one at the top each other 1,920 x 2,205 at 24fps (black gap between 2 frames).
Interlace mode should be done in the player (computer or video card) to upconvert and produce 3840x2160 line-interlaced 3D video stream.
As long as you don't have some sort of overscan and scaling going on with the display showing 1 to 1 pixel ratio you should be able to see correct 3D with passive glasses and 4K display.

Is that what we are talking with interlaced mode? If interlace mode is same kind of 1080p with both frames interlaced than "hardware mode" 3D MVC frame packing has to be better.

Maybe you should be asking; "What is the difference between 3D MVC 1080p Hardware mode and 3D 4k Interlaced mode?" and as a next question "is the difference also visible?"

Edit: Maybe I should remark that my HTPC setup is putting out a 4k signal to the receiver when playing a 3D BR iso. No up-scaling is done by receiver.