I have encountered recently the similar issue - "unpleasant amount distortion and sibilance heard on vocals" -though, in different system (Rega P25 table, RB600 arm, Denon Dl-103 cartridge).
Went down googling last night, and found a discussion of smilar problem with dl-103 on rega.
While I thought my cartridge was aligned perfectly (I am using default Rega protractor), actually it was not, I had to move the cart all the way front on arm and align it there (before its was aliged somewhere in the middle). it worked - sibilance on vocals now seems gone.
the questions is if you hear sibilance on the whole LP? My problem was starting with like second or third track on one side.
So #1 check the cartridge position on the tonearms end. #2 if the problem seen on all tracks, that could be defect of of cartridge (factory re-tip is quite affordable for benz Micro LP, $800 at musical surroundings). #3 might be silly idea, but it could be tabe/arm and cart mismatch (key word hear compliance??)?
Just my two cents...