It's a stretch, but if you have your HDD connected, try unhooking it and starting up. Never would have thought that before yesterday, but I had a similar scenario to you, and after breadboarding (again) with every possible combination of two motherboards/processors/power supplies/memory that I had laying around. I also was testing with the HDD attached. As soon as I disconnected the SATA power, it would post. Without it, with some combinations I was getting partial a fan turn on the PSU and CPU cooler, and then nothing but the standby light.
For me, it was specficially the SATA power cable from the PSU, itself. Swapped out to the other one that came with the PSU, and it started right up. Unfortunately, the presumed short that started all of my troubles (a pop and a whiff of smoke), was elusive to me for a couple of days, as it was after burn-in and hooked up in my entertainment center. Started getting on the right track with that yesterday when I hooked up a "burner" (pun intended) HDD from an old system pull that was ready for the trash pile. As soon as I hit power, I actually saw a spark and a small flame at the junction where the power cable is attached to the drive. Thankfully, the new drive is under warranty.