I'm usually solving other people problems but I can't find a solution to this one.
I got the X299 Dark yesterday (just wanted to play around since the recent price drops) and at first, I had constant A1 memory slot error. Somehow it passed after I removed all RAM from other slots and used only A1 slot. I have no idea what was that and I hope it won't back. Now there is some other issue which I noticed. Memory write, no matter how I set my RAM, is almost always at 50% expected performance. For example when I set the memory at 3000 then I get something like ~58GB/s read, ~55GB/s copy but ~32GB/s write. The same problem at auto, XMP or manual settings. Results in AIDA64.
 
So far I've checked 3 BIOS releases (including the last one). Higher SA/DIMM voltages and mentioned auto/XMP/manual memory settings. This issue usually happens when sub-timings are wrong but at SPD profile everything should work fine. When I set auto/SPD 2133 then write bandwidth is also slightly above 30GB/s. When I set higher mesh clock then read/copy go up, write not really. Then is something like 60GB/s read/copy and 32GB/s write.
 
I had only one quad kit for tests yesterday but will check it on some other kits during the weekend.
Used memory was 4x16GB Ballistix Elite 3000 / Samsung B-die. The same memory was tested on at least 8-9 different X299 motherboards in the past.

Not yet, will try it later, thanks.

I thought I did that already but will check again later today. Thanks.
On other mobos the same was happening when BIOS was setting timings automatically at 4000+. Then simple manual settings were solving it. Also on SPD/XMP profiles were no issues while now I have it even at 2133 what is weird.
This is my first EVGA mobo so I still have to spend some time on various settings.

Woomack just for clarity what is the exact part number for the kit you have. I want to look it up when I get home from work this evening

Woomack my apologies. I had posted the above while looking at the EVGA forums on my phone during lunch break and for some reason I had it in my head you were also having the XPM speed issue posted here - https://forums.evga.com/FindPost/2938936
 
As for the difference in quad / dual channel the increased throughput is expected, this board really does work best with all 4 slots populated

I was under the impression a person was to run it with the relearn set till they were happy (reboot / test a few times) then turn it off. Then do any final tweaking if needed.

Woomack
I was looking for an issue in the motherboard or memory settings while it was the CPU. I got unpacked but new 7800X the same day as the X299 Dark. Today I switched CPUs with my other rig and exactly the same happened on the other board. This CPU limits memory write bandwidth for some reason. It's really weird and it was the last thing I would expect. Have to return it to the store on Monday.
 
Thanks for your time reading and answering to my posts. I can finally start to play with the new mobo. Will post some results later.

Yea, the CPU heavily impacts the memory on this platform. It is not enough of a lottery for a good overclocker but also have to factor in the IMC performance as well

Interesting.

Here is the 2nd part with the CB15 tests @3600,3733,3800,and 4000 <- with this score being lower than the run @3800
 
Thank You

I am watching this ATM   The Travelers Episode
Quad Channel Memory Runs Better and Overclocks Better than Dual Channel Memory on the X299 Dark Motherboard.
That is you will get better Memory Performance running Quad in the end.