New PC, no POST

[dystopian]

So, I recently finished building my first system, and the CPU power headers do not work (either with switch or shorting). Specs as follows:

• Ryzen 5 1600 (with Wraith Prism) - Possibly issue
• R9 390X 8GB - 99% sure FF (fully functional)
• 8GB Vengeance LPX DDR4-3000 - FF
• MSI B450M Pro-VDH MAX - possibly issue
• Seasonic S12III 500W 80+B - FF
• Phanteks Eclipse P400A DRGB - FF

My current symptoms are:

 

System does NOT boot or POST when power switch is pressed, nor does it when the power switch headers are crossed with a screwdriver (or other metallic objects such as detached multimeter probes). Nothing happens in any of the above situations (no fans spin, no error lights). When I jump the PSU (detaching the 20-pin mobo header but leaving 4-pin in), all fans spin up (except GPU) and all fan RGBs come on. The MSI EZ-Debug LED for the CPU comes on and is solid white. The same happens if performed after a power drain and CMOS reset.

 

Though it may no longer be relevant, I have already replaced both the CPU and the MoBo. CPU was replaced with the same model, and motherboard was originally MSI B450A Pro MAX, but replaced with MSI B450M Pro-VDH MAX. Before, I had a similar but different issue, as descibed: 

System does NOT boot or POST when power switch is pressed, nor does it when the power switch headers are crossed with a screwdriver. Nothing happens in any of the above situations (no fans spin, no error lights). When I jump the PSU (detaching the 20-pin mobo header but leaving 4-pin in), all fans spin up (except GPU) and all fan RGBs come on. The MSI EZ-Debug LED for the CPU comes on and is solid white. If performed after a power drain and CMOS reset, however, all fans spin an the system attempts to boot. However, it would give an error 'USB Overcurrent detected, system will shut off in 15s'. Once, I did manage to get into BIOS and begin the windows installation sequence. It worked, but upon automatically rebooting (part of the installation), the USB error reappeared. I have never been able to replicate this result.

 

I am certain that the RAM and PSU are fully functional (both tested in other PC, which is sadly Intel so no testing CPU or MoBo). The GPU I am almost certain it functions as it outputted video correctly when I managed to get it to boot that one time with old CPU/MoBo.

 

Does anyone have any advice on what I can do? Are there any troubleshooting steps I've missed? (I have read through the 'no POST' pin.) Will I have to return something? Thanks for reading all of this, all of your help is greatly appreciated.