I replaced my squealing Avid Shorty 4's with the Shimano BR-R550 Cantis and I have finally and permanently eliminated the awful noise. Couldn't be more pleased with the BR-R550's. I believe that the Shimano brake shoes may make the contact between pad and rim more stable and prevent the vibrations causing the squeal. I had been using the Kool Stop salmon pads on the shoeless Shorty's, and the Kool Stop's had not helped in lessening the noise.
The stopping power is precise and powerful.