40 years of fitting cartridges to a variety of arms, I'm feeling stumped

You can be off by quite a bit and it will not result in such gross problems as distortion and excessive sibilance (evidence of mis-tracking).  It is most likely problems with the cartridge.  Two arms, other cartridges working with the arm, what else could it be? 

I've only heard this with a healthy cartridge when the arm was extremely cheap and crappy--my guess is that the whole thing was resonating like crazy.  The owner wanted to temporarily use a really cheap table with his new Ortofon PW cartridge.  The combination didn't work and there was excessive sibilance toward the end of the record.  A cheap cartridge worked in that arm with no problem.  The expensive cartridge probably has lower compliance which made it less compatible, and because lower compliance high end MC cartridges tend to feed a lot of vibrational energy into the tonearm, the arm must be up to the task ((rigid, good internal damping, tight bearings that don't rattle, etc).

My only guess, given your skills, is anti-skating. 

How do you set it? Anything seem different with this cartridge?

You could try using inner grooves to set it, then play, problems on last track persist? That will tell you something.

Also, given your skills, I agree, send it back to them, have them check it out.

I'm using a Soundsmith cartridge on a 309, and did run into an issue that affected the last track or two on a side (though it was mistracking rather than sibilance and distortion). I was having to lower the arm to get it level to a point where the arm was touching against the arm lift, even when it was lowered. The SME manuals say their arm lifts are adjustable with a tiny Allen key inserted in the tiny hole in the middle of the rubber cap on the arm lift, but I cannot find such a hole on any of my SME arms. I shaved off the ridge on the rubber and all is well.

Looks like The Voice is sored by COVID19 and in need of some medical attention.

Take The Voice to a good ENT specialist.