L channel distortion--can't resolve by AS/azimuth

I'm trying to sort out a cartridge issue. My cartridge is a Clearaudio Maestro, purchased new a few months ago. I'm getting sibilance & distortion in the left channel only, usually on transients/loud passages. It is most notable on vocals and brass.

I have used the manufacturer-supplied protractor to set up the cartridge and the Hi-Fi News Test LP to set anti-skate. I have adjusted azimuth to varying degrees in both directions, but I still cannot seem to get rid of the noise.

I appreciate any suggestions.