208v isn't low enough to toast those PSUs, but running them on an-regulated voltage can easily kill them, depending on the electity infrastructure, voltage drops could be so sever and that 208v point might not have been there all the time, there is a chance that you got a bad batch if that was the case we would be hearing more complains from more people, I am fairly active on all mining groups accross all things internet (this forum, telegram groups, FB, and etc.) bad/low quality batches are usually heard off in a short period of time, so I tend to kind of negate the batch issue, because there is noway that they would make only 6 bad units.

So my thought which is the same as the initial one is that the PSUs are causing the issues, I havn't looked inside a Whatsminer PSU but I am pretty confined that all new PSUs are pretty hard to fix or at least hey were not made to be fixed, but there is always hope, what I would do is get a used/new PSU and test it on all those units, for obvious reasons in somewhere where you know your voltage is stable above 200v.