What do you mean by "precompressing the bearings"? Do the bearings spin freely prior to installation? Do they spin freely after being installed in the bottom bracket shell but without the crank installed? Did you have your bottom bracket shell faced prior to installation?

Looking at figure 1 in this PDF (http://www.stronglight.com/stronglig...kgauche.an.pdf), are you installing items #2 and #6? I'm not 100% certain, and would need to disassemble my crankset to find out, but it's possible that #6 prevents the bearings from being crushed during assembly by supporting the inner race of the bearings.