Followup/resolution:
So after tweaking my alignment ad nauseum, I wasn't able to reduce sibilance distortion. One day I happened upon this massive thread:
http://forums.stevehoffman.tv/threads/many-lps-are-incompatible-with-normal-cartridges-cant-be-played-without-sibilance.255701/TLDR: the OP claims that certain catrdiges cannot track certain records, particularly modern records. I was skeptical until I read that the OP claimed Peter Gabriel's "So" could be rid of sibilance distortion with a particular cartdridge. I have that record and it sounded AWFUL on my system--sibilance distortion made it unlistenable.
I read the entire thread and wrote down all the recommended cartridges for sibilance distortion elimination:
zu denon dl-110
ATMLA440a or AT440MLB
Jico SAS stylus with ShureM97xE
AT150MX (AT150Sa)
KAB Stanton 681 EEE with the Pickering D11 stylus
Nagaoka MP-500
I decided to buy an AT440MLB because it was the most affordable. I set it up on my system and... fixed. Every record I play now has perfectly clean sibilance. It's like magic.
One downside is this cartridge is very bright. I'm hoping that it will mellow with age. I appreciate the extra detail it brings out, but I miss well-roundedness of my Stanton 680ee.