Grabbed a second 700W FSP700-80GLN (er, the warranty replacement for the first one !), so I'll have a fiddle around tonight when I get home.
This is what puzzles me the most : Modern PSUs have protection against shorts, overvolting, overloading, etc. etc. - if anything bad happens, they just switch off. I'm struggling to understand what could actually be blowing the PSUs when they're under load - they surely should just be switching off ?
...which makes me think about what lemonadesoda says above - maybe the PSUs are dying because of some issue with their input, rather than what the PC is doing with their output. I do actually have about a gazillion devices all plugged into the same double wall-socket...