X370 Killer SLI/ac won't POST

I'm a generally experiened PC builder but this is my first time with Ryzen. Components are:

Board: x370 Killer SLI/ac
Chip: Ryzen 7 1700
RAM: GSkill FlareX F4-3200C14D016GFX
RAM v2: Patriot Viper Elite PVE44G240C6GY (2400MHz)
PSU: Corsair HX750 (old part known good)
GPU: NVidia Strix 1080 (old part known good)

What happens:
When I turn on the system the mainboard LED pulses red, and no video output.

Things I have tried:
*Swapping between the two RAM sets (on the theory that I need a BIOS update to use the 3200 RAM)
*Going to a single DIMM in the A2 slot of the GSkill RAM
*Resetting CMOS
*Moving the GPU between the first and second PCI slots
*Switching to an ENTIRELY NEW x370 Killer SLI/ac board (I purchased two boards and two CPUs because I plan to do an identical build for my wife's PC, so I have a second board onhand)
*Inspected the CPU for bent pins when I moved it to the new board
* Installing a PC Speaker by connecting it to the PC Speaker pins (no beeps resulted)

Help please!

Thanks,
Michael

We made it work. Couple of things it could have been:
1) a hard drive needed to be plugged in?
2) First POST takes a long time and I was never patient enough. I may have accidentally left it on long enough one final time and I came back to the room and it had posted.

Thanks!

Ok, I have a new problem. I am starting on the second Ryzen build and again I'm running into a lack of POSTing. The specs are the same but recall from above that when I tried to get the first system to POST, I ended up swapping out the motherboard and using the second x370 Killer SLI/ac motherboard I purchased.

Now I'm trying to get the second system to POST, using the first motherboard (which never POSTed successfully, although the second one did).

So the current status is:
Known good RAM (I used it to POST the previous Ryzen build).
Known good power supply
Known good GFX card (ASUS NVidia GTX1060)
Known good monitor on HDMI

The only two unknown pieces are the Motherboard and the chip.
I checked the chip for bent pins.
I have the 24-pin and 6-pin connectors securely feeding power to the motherboard.
I have a 6-pin connector securely feeding power to the GFX card.
I have cleared the CMOS after inserting all the pieces.
I have the RAM in the right place.

I have waited upwards of 30 minutes at a time for POST with no luck.

I think the only thing left to do is RMA the motherboard, yes?

Sorry, yes. Mistype. The right lead is going to the CPU. I know the difference between the two.

I RMA-ed the board and got a replacement.
The replacement POSTed immediately.

So that's good. Now I have another problem.

The RAM I have for both builds is G-Skill FlareX F4-3200C14D-16GFX.
I have the BIOS on the second machine as v3.50 (the first machine has 3.40).
On the second machine, setting the RAM to its XMP profile (CL 14-14-14-34, 1.35v, as listed on the package) results in serious instability and constant BSODs. Setting the RAM to the 'default' of 2400 appears to be stable.

I have tried swapping the RAM between systems, and as far as I can tell, it's not the RAM. The same problem happens to the second system when I put the (identical) RAM from the first system in the second system.

Not sure why this is. The BIOS v3.50 should definitely be able to handle the 3200 speed RAM. The first system has been steady at 3200 speed, and it has BIOS v3.40.

I don't think it's a driver issue, since the BSODs are pretty random types, all over the place.