How can I preserve the quality of terrain, when converted from Mudbox to Unity?

Here's how it started:
I sculpted an 8km×8km terrain in Mudbox, exported hi-res terrain mesh to a modeling software, rendered a heightmap.
When I imported this heightmap in Unity, it looks terrible. I tried tweaking pixel error. Normally you wouldn't set pixel error to 1 since this makes rendering terrain slower. But even if I set this value to the lowest value possible, It still looks bad, not even close to the original.

What it looks like in Unity, with a global normal map and a global color map applied:

https://imgchr.com/i/jvYSx

What it looks like in Mudbox:

https://imgchr.com/i/jvGf1

In case both pictures are unavailable, see the file Screenshot.zip attached to this thread.

Notice that when I was sculpting this terrain, all brushes are made constraint to Y-axis, which means the heightmap should be able to represent what it looks like in Mudbox theoretically.

Any ideas on how can I make this terrain look better and not so low-res when pixel error approach is simply impossible?

 

This is a topic I know little about - but here are a couple things to check.
1. Why not export heightmap directly out of mudbox?
2. What is the resolution of the height map?
3. Without a similar light/shadow setup in Unity the results will always look like a lower quality result. Set up a lighting setup similar to the lighting in Mudbox so visual comparison is equal.

 

1.I don't think I can export heightmap directly from Mudbox, maybe I just overlooked this option.
2.The polycount of this terrain is 1,638,400, which is equal to 1280*1280, so the resolution of heightmap was set to 2k.
3.I said the terrain in Unity looks low-res because it looks blurry (and therefore low polycount), and I don't think that has anything to do with the lighting setup.