Maybe your guitars all share a certain tonal characteristics (and that is the case judging by the recordings that you have published in viewtopic.php?f=11&t=111723&start=255#p1214113)and
that could entail a strong top resonance around these bass notes making unduly big excursions of the string. I have this in one of my guitars, and this is what I have to live with if I want to play this guitar, because it is her character... I could raise the action, but it would have to be a lot and I don't want to do it.
One can also take a littly ball of putty and experiment with placing it on the bass side of the bridge and oving it around to see if the buzz gets less. If that would be the case, you could put a similar ball of putty at the same place uderneath the top. But this will also change slightly the tone, so one always has to find some compromise. A slightly higher string tension ar thinner but stiffer strings at the same tension could also help.
Sometimes it also helps to make the string slightly stiffer by twisting it axially so that the windings get a bit closer together, but one can easily exagerate and ruin the string (You have to find out which way the string is wound: clockwise or anti-clockwise!)
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