Hi Jermy!
Someone else had the same issue as you: [post|633237]
My guess was the power supply too.. But you already replaced the power supply. So this should be good.
What more to test:
* Harddrive
* Wifi board
* Bluetooth board
* Last thing: Check the motherboard and meassure if possible
You have another Xbox X (which works) there, right? Just try to swap parts out and you’ll see what part is causing it. I know it sounds kinda lame, but there are a lot of parts which can cause the xbox not to boot due errors.