Brand New RM850x Dead

Edit: Never mind the power supply was NOT dead and probably working just fine! It was an issue with my video card, not the power supply at all.

Hey all, My PC is dead so I have plenty of time to sit around and write about this problem so bear with me.

Quick background: I just put together a new Ryzen 7 system that after RMA'ing a (seemingly) dead Radeon RX 580, I started getting coil whine from the power supply. I went to Best Buy and bought a Corsair RM850x to replace my dying PSU. Installed the Corsair PSU, and tucked all the extra cable away and we're off to the races. No coil whine, PC is running like a dream at this point.

Fast forward to last night, I just got done updating my Radeon drivers to 19.1.1 and I'm watching youtube and browsing reddit. I decide to fire up a game of Heroes of the Storm to see if the new drivers had any noticeable improvements. I get into the main menu and start to feel confident that everything is running smoothly (sarcastically laughing to myself about the problems I've encountered after building this new PC, but those days are gone!... right?) I notice a brief graphical glitch in the background which makes my heart stop, and then suddenly everything is turned off in an instant and I'm sitting in silence.

Through my years of troubleshooting and building computers I instantly knew it was a power issue. I popped the case open and had a look at all the connectors and cables to see if maybe something came loose, everything looks great. I thought maybe I'm drawing too much power with the RX580 using two-6 pin connectors into an 8-pin dongle, so I ditched the dongle and just direct connected a 6+2 connector straight from the PSU into the RX580.

Crossing my fingers, I delicately re-connected my computer and lightly pressed the power button. I hear a couple of fans begin to spin, and a flash of some RGB lights and then it's off again. Not trying that again.

I bought this RM850x power supply less than 12 days ago and its toast. How? I'm not overclocking anything, and its well within the boundaries of power that my PC would draw even at heavy load. What are the chances that my whole system is fried with it? I'm extremely nervous about this, as I built this entire PC less than a month ago.

I'm going to return the PSU tonight and hopefully get a different brand PSU but it seems like Best Buy only carries Corsair... Is it worth it to just get another Corsair PSU or hold out for an EVGA or something?

Update:

Well it turns out it's not the power supply after all....

I replaced the RM850x with an EVGA 850 GQ last night and I'm still experiencing the same issue. Everything powers up for a split-second and turns off. I have RGB Vengeance ram that stays lit up while everything else shuts off.

At this point I'm desperate for suggestions. I haven't had this many issues with a newly built PC in my life. And now I can't even get it to turn on.

I'm going to try breadboarding my PC tonight or tomorrow to make sure the components still work... I'm leaning towards a motherboard fault but I'm really not sure right now.

Final Update:

Oddly, it seems to be my newly RMA'd RX 580 that caused this problem. I pulled the video card out and it booted right up no problem.

I'm absolutely stunned that a freshly RMA'd card from the manufacturer only weeks ago is already dead. At this point I think I'm **** out of luck and have to buy a brand new video card. Yay.