I was working through testing the ram sticks and had got to ram stick 1 in slot three, which again gave nothing I could discern. No boot, but the led on the power button started on continuous white, when boot failed it changed to yellow flashing in groups of 5. I switched off by depressing the power button for several seconds until the LED went out. I left the room to make tea for my wife, heard a loud bang and from the kitchen returned rapidly to find the smell of burning coming from the computer.
Can you recall what the power button LED was doing before you started any troubleshooting, this includes before you first removed the CMOS battery.
I suspect that this has been a PSU problem from the outset and in no way suggesting anything dishonest but I would recommend that you make sure that the RAM is returned to the slots that the sticks were populating when you took delivery of the computer.
You are welcome, keep us posted and good luck