Just picked up a Nighthawk X6 R8000 over the weekend. It is mounted on a wall in the living room on the first floor of our 2 story 3600 sq. ft. house. I have the six antennas flipped out as as far as they go. The top two and bottom two antennas also articulate up or down. There is nothing in the manual about the best positioning for the antennas when wall mounted. Has anyone found an optimal position for them?
Yes, you need to experiment as stated.
However, is the wall it is mounted on an exterior wall, or an interior wall? If exterior and you don't want maximum coverage beyond it, ok. If interior, it could be causing issues no matter how you position the antennae.
I would recommend at least 10 feet (3 metres) location above ground level (not floor level). No obstructions for 3' (1 metre) all around. This would be an ideal placement (but not many decors allow for this).
And the router (or AP) should also be placed in the center of the area to be covered (when the area is viewed in 3D), keeping the above restrictions in mind. Small (or large) adjustments can and should be made to find this 'center' as needed. Including adjusting for line of sight (or lack thereof) to high bandwidth requirement devices, differing construction materials and the relative importance to certain areas and/or devices usually used in specific sections.
I would also run
at least 5 LAN cables to wherever the main router is (and with the RT-AC88U;
at least 9 cables to feed all the WAN and LAN ports properly).
Configured properly, the antennae orientation, while it can make a difference depending on the environment specifics, is the last thing to worry about.