I had the same issue yesterday, I had a brand new 430w EVGA gold PSU and it wouldnt start my new B250 Expert. Grabbed a spare dell 1100w PSU and boom it started right up (and yes you need to short the PWR_SW pins on the mobo to start it). The pins are on page 1-11
http://c3.clewm.net/BQnEfp.
I think that the PSU I had was just defective. I shorted the purple to green on the PSU and it started but couldnt get proper voltages from it.
Also from a ram/CPU prospective, it depends on the mining programs and OS. I'm running windows 10 (full update to get the nvidia drivers to work) with 6x 1070 ti, 4x RX580, 2x RX570 with a dualcore Celeron and 4gb ram. The NVidias use 1% CPU and 25% ram, but when I turn on Claymore for the AMDs, my RAM spikes to 95% and CPU to 100% and my page file adjusted to 6gb which is ok since its SSD but still better to run in ram than HDD swapping. I've temporarily shut down the AMD string and will be checking the risers and connections. Either I'm getting a PCIe loop on one card thats taking all the CPU cycles, or the overhead is just high with that miner. I'm going to do more testing, but I'm considering jumping up to a quadcore i3/i5 and 16gb ram to get it stable if risers dont solve the issue as I'm looking to add 11 more card to the rig and it definately wont handle it with the AMD load that high.
As a side note: I dont power the GPUs from the mobo, I use a separate server power supply and 6pin breakout board so I cannot comment on that aspect of the board.