I've discussed this issue not only this time around, but before with others, as to why there seems to be no clear cut answer as to why sibilance problems are noted, and have now all of a sudden seem to be commonplace with a JMW Arm.
I don't recall ever reading such in the past, and the JMW seemed always praised as a very good Arm. So what changed?
Years ago, when I was using nothing but MM Cartridges, and some fairly decent AT, and Shure Models, I never seemed to notice much in the way of artifacts, and in truth, I never had a protractor, just those miserably cheap $2 plastic Stylus Alignment Gauges. Never noticed much in the way of break in either with these. They sounded good fresh from the box, and changed little as they got some hours on them.
But enter the MC, and here's where things started to become complicated. The amassing of tools of all sorts, and the OCD setting in.
Even with my own ZYX Airy 3X fresh from the box, I experienced some sibilance on certain tracks, as one example was Tony Bennet spitting a little too much into the Mic on the tune "San Francisco".
It was my error though, erroneously thinking I could follow some other poster's recommendations of setting VTF to 1.86g right off the bat, with a fresh out of the box Airy 3 Cartridge. Increasing VTF to an even 2.0g solved that problem when the cartridge was new.
And as others here have taught me, to not ever assume what might work for others, especially as an optimum VTF setting, will then optimally work for you.
These MC's will need some break in time, and about all I can add, is analog is a medium which of course is not plug and play. Something may be assumed to be correct, such as a Stylus Force Gauge, but can be something that is overlooked, and in error. One needs to confirm accuracy of such tools, to at least know if you are setting the Cartridge at say 2.0g, that it is truly indeed 2.0g, and not some huge error, like actually being 1.5g.
That although one should trust their ears to optimum settings of VTF, one still should know accurately that they are in the ballpark range of VTF per manufacturer's given specs. I would bet many have been fooled with less than accurate set up tools.
This is just one example I cite. There can be others as well. Take VTA as another example. I too have been fooled by thinking I was at a neutral VTA, and was actually quite far from it. While not all cartridges may optimally work at neutral VTA, it should be a baseline from which to go from.
Mark