This is a good question, as I too am not listening as much to some of my more vocals pop/modern/reissue LPs because of the sibilance of the Master Signature. I also hear the issue on some jazz reissues with muted trumpet and cymbals. However, there is absolutely no sibilance with choral music, massed voices, or soprano/tenor solo on my original classical LPs. I am beginning to question whether the issue is with the cartridge or with a few specific recordings. Otherwise, the cartridge is glorious and the best my system has ever sounded.
Apparently, when I send the cartridge back to vdH for its 250 hour check up, the high frequencies above 20kHz can be somewhat attenuated. I don't understand how this is done, but I am considering this option. Otherwise, if my second arm project (custom outboard armpod with SME 3012R arm) ever gets finished, I will have two arms on my SME table and will be able to use my MSL Sig Gold on one arm, and my Master Signature on the other, as is desirable for different music and recordings.