My specs:

ASRock X370 Killer SLI/ac

AMD Athlon X4 950 Processor (old part, nkown good. for testing purposes)

HyperX Predator 16GB (2x8GB) 2666MHz

XPG SX8200 480GB 3D NAND NVMe Gen3x4 M.2 2280 SSD

Corsair Hydro Series H100i v2 AIO Liquid CPU Cooler

EVGA SuperNOVA 550W G3

I'm not sure what the GPU is exactly - I'm borrowing it for testing purposes, but it's known to be a good part.

I've tried:

Placing one stick of RAM in each slot individually, clearing the CMOS between switches, to no result. I've tried this with each different set-up I've done to try to make it work.

Leaving the CMOS battery out overnight 

Switching from my SSD to a known working hard drive

Moving the GPU to the second slot

A different PSU

A different fan for the CPU

Different HDMI cables

Different monitors (a known working TV and a known working monitor)

But none of this has worked so far. I also checed to make sure whatever I changed out was seated properly, including cables. I have also replaced the mobo once, since I thought it was the broken part, but that doesn't seem to be the case (maybe).

All I know is that the MOBO beeps when there's no RAM (3 beeps, which seems to be accurate to the error code lists I've seen for "no RAM") but it doesn't beep when there's no GPU in.

Any advice or help would be very appreciated!