The render looks good. When I played with Quixel materials, here is what I did:
diffuse: Corona Mix with a mix of diffuse map (albedo) and AO map (multiply or overlay, I don't remember :S )
reflection: specular map (with gamma 1.0)
glossiness: gloss map (with gamma 1.0)
bump: bump map (with gamma 1.0)
displacement: displacement map (with gamma 1.0)
Then I adjusted reflection amount and fresnel IOR to make it look more or less ok.
I also adjusted displacement height to make it look good.
You can then play with displacement quality in render setup, but usually the default value of 2px should work fine.
That looks good in all honesty..
Just to add to your question? Should bump,normal and
displacement maps always be set to gamma 1.0
No. It depends on the file. You should either use what looks good, or what looks correct. :)
Here is an example:
Max scene:
Results: