I'm confused... why on earth with new cpu, mobo, memory, and gpu it would not post kills me.
Two things I noticed:
1. Swapping out one part for another is a viable method of troubleshooting...but you took it to a whole new level. I'm afraid that replacing so many parts, and swapping so many GPUs in and out, have only confused the issue. Not to mention that with each motherboard swap, you increase the chance of wiring it incorrectly, or not seating a component correctly.
Start with the pieces you want in the final build; then replace them one at a time. If you swap out an item and the problem persists, re-insert the original component and move to the next computer part. Also, while troubleshooting, you should reduce the RAM to one stick.
2. This is only a guess (so I could be way off), but your success (and failures) sound sporadic. What's the one component that has stayed consistent through your troubleshooting? Sounds like the PSU. And a failing PSU can be sporadic. So if the problem persists, you should consider the PSU as the culprit. I realize Seasonic is an amazing brand, but my experience has told me PSUs are the single-most likely component to fail.
Hope this helps.