I think the PSUs are having some issues indeed, but it is really strange that the one always fails is the PSU that is not powering the controller board.
Thanks, I'll think I'll go change the PSUs on Monday.
Turns out the VX450s only have 400W of power on the 12V rail, such that the addition of the controller would put them over the max. Its also not great beacuse it puts 350W through one PCI-E cable which is putting the cable way over spec.