Poweredge R730Xd strange power issues

I purchased an R730XD a few weeks ago, and it's been working great... until one day it shutdown.

Trying to power it back up, it whirs up for a second, then shuts off. iDRAC is saying BP1 voltage out of range.

A bit of research, and it indicates that's a backplane issue, so I disconnected the BP, and the server fired up. Great, so I ordered a new BP.

I've just installed the BP, and the server still wont fire up and gives the same reason. I unplugged one of the power connectors from the BP, server fires up. Plugged it back in, disconnect the other power connector to the BP, server fires up.

The only thing that's changed since I purchased the server, is I put in 2 CPUs (2660V4 replacing the 2650V3s)... but I changed the CPUs on day 1, and the server was workign fine.

I've tried a 750W (same as the Wattage the server came with) PSI from a R720, and still has the same issue.

No other devices connected, no PCI cards or anything.

Does anyone have any ideas? I'm at a bit of a loss... short of buying a 1100W PSU and/or a new mobo...