Downgrade packages using apt-pinning

The apt_preferences manpage says that you can downgrade packages by specifying a priority above 1000. I'd like to downgrade all my packages to the stable distribution. My sources.list contains stable, testing and unstable.

This is my apt.conf:

APT::Default-Release "stable";

This is my preferences file:

Package: *
Pin: release a=stable
Pin-Priority: 1001

I tried apt-get update and apt-get -s upgrade but apt does not suggest downgrading. For example I have the version 1.18.0-3 of libuv1 installed which is newer than the current stable version in stretch which is 1.9.1-3.