My AT150MLX Cartridge has been misaligned for 4 years

(but I still get some sibilance and harshness at loud peaks

You’re using a Rega 3? The arm has no VTA adjustment which is a challenge dialling in a cartridge with a fine profile. It sounds like it could go a tiny bit lower at the pillar end. You could try a shim above the cartridge. 

Yes P3 I do have the ringer spacer that Rega makes.

my overhang was too forward on the tone arm when I first got the TT I downloaded from Vinyl Engine a Stevensons protractor.

Stevenson is more backwards than others.

When you download and print something in the wrong size (it is always happens with home printers) then you are totally off. 

Actually I put the pdf on a thumb drive and brought it to kinkos to have them print to scale and laminate it LOL . I think this was my mistake because there are lines that you need to make sure are pointing at the pivot point of the tonearm I failed to do that.  The AVID protractor has it listed as the first thing to do.  Still spinning and all seems much better.

Thanks again follow up :)

Hope you all are staying safe during this crazy time...