Screaming brakes

I've got a Masi CX that I sometimes commute in... the thing that puts me off riding it is that the front brakes SCREAM if I hit them hard-- enough so that I scare pedestrians, which isn't good on an urban commute. The brakes stop just fine, and at 50% or less are silent; it's only a problem if I hit them hard

I've done everything I can think of-- toe in, adjust, various kinds of theoretically snazzy brake shoes (it's got Kool Stop Triple Compound pads at the moment, and the brakes are cantilever). Last week, I finally gave up and took it into the LBS. $20 later, it's exactly 0% better.

Anybody got any non-obvious suggestions? It seems to me that there's a problematic resonance between the fork and the rim, and that it may well be unfixable