The plot thickens .. I came across a few threads on the online forum (the name that sounds like 'Beer Guts'), where one of the posters (who is, I believe, a 'reliable' recording forum / online figure known these days for his equipment modification services in the 'real world') talks about doing tech work on the Group IV studio T*****t A Range...
An A Range would make sense too given the owner Dennis Sand's prior career using an A Range at Cherokee Studios... A Range - a legendary console considered up there with the best of the best (!).. only 13 ever made.
The prior advice to roll off some highs seems good if you are faced with an overly sibilant vocal.. (though paradoxically a classic way to add 'air' to vocals is boost ultra highs like 12-15k) .. also a tried & true technique to deal with problems is if you have any eq with a bell shape you can boost while you sweep or click through the frequencies to find the most problematic / annoying spot and then reduce at that point.. one has to take care with with vocals & eq/de-essers etc as the human ear is (unsurprisingly) most sensitive to frequencies in the range of human speech so vocals can quickly start to sound strange the more they are tinkered with.. sorry if this is all old news (!!)
Here's some handy articles with tips & strategies on how to deal with (and avoid in the 1st place) vocal sibilance..