Z170M motherboards won't POST

I have the following kit:

Corsair AX1200 PSU (used in all tests below)

"Old Build":
Corsair Obsidian 650D case
ASUS P8Z77-V mobo
Core i7 3770K

"New Build Try 1":
Gigabyte GA-Z170MX-GAMING 5
Core i7 6700K with Cooler Master Gemin II M4
With 16 GB DDR4 (1 stick - tried in all four slots)

"New Build Try 2":
MSI Z170M-E D3
Core i3 6100 with stock Intel cooler
With 8 GB DDR3 (1 stick) stolen from Old Build

With the 24-pin and 8-pin power connectors plugged into the same slots on the PSU sitting on my floor, I tried the following:

1. Boot up the Old Build by connecting the 24-pin and 8-pin power to the mobo, HDMI to the integrated graphics back panel connector, and pressing the power switch

Result: POSTs just fine. I see the BIOS screen within seconds.

2. Bench test the New Build Try 1 by connecting the 24-pin and 8-pin power to the mobo, HDMI to the integrated graphics back panel connector, and shorting the power switch with a screw driver

Result: Fan spins for about 12 seconds, then the PSU and CPU fan cut off, a "click" is heard, the motherboard appears "dead" (LED goes off), then after 4-5 seconds it comes back on. Rinse and repeat forever.

3. Bench test the New Build Try 2 by connecting the 24-pin and 8-pin power to the mobo, HDMI to the integrated graphics back panel connector, and shorting the power switch with a screw driver

Result: Fan spins indefinitely. Motherboard LED indicates continuous power is being applied. It will remain in this vegetative state, not displaying anything on the screen, for as long as I leave it powered on. It never turns off of its own volition, unlike the Try 1 kit. But I don't get any video either.

My "bench" is a bunch of printer paper spread out over the floor. Seems not to be a short, since any short would cut power to the Try 2 motherboard, but that never happens.

I tried resetting the CMOS by shorting the pins and taking out the battery for both motherboards.

I tried reseating both CPUs.

I tried swapping coolers.

I tried putting the RAM in each and every slot in both motherboards.

I'm at wit's end. I'm at a loss as to why this is happening. I read through the "read before posting" guides here and nothing seemed relevant or particularly helpful that I haven't already tried.

I have checked and triple-checked that my PSU still is not bad by successfully POSTing my Old Build at-will. That system has been rock solid stable and has recently had uptimes exceeding 1 month (with frequent heavy gaming), so there is no validity to the hypothesis that my RAM or PSU might be bad, especially since the Try 2 board uses a RAM stick from my Old Build, and that RAM is known good.

There is also no validity to the hypothesis that the CPU is overheating due to improper cooler installation. The stock cooler install is ridiculously easy, and the motherboard stayed powered on and the CPU fan remained lit on the Try 2 build for upwards of 45 minutes without displaying anything on the screen or resetting or powering down. If the CPU were overheating, the motherboard would power down and the fans would stop spinning.

Do I _really_ have _two_ DOA motherboards or CPUs? Or is there something wrong with my test bench or procedures?

This stuff always seems to happen to me Never lucky...

All ideas appreciated, but please don't ask me to do something I've already done per my list of things I've tried above. Thank you.