Configuring Amavisd and SpamAssassin

Enabling SpamAssassin
Edit /etc/amavis/conf.d/15-content_filter_mode

Uncomment @bypass_virus_checks_maps = (   \%bypass_virus_checks, \@bypass_virus_checks_acl, \$bypass_virus_checks_re); and @bypass_spam_checks_maps = (   \%bypass_spam_checks, \@bypass_spam_checks_acl, \$bypass_spam_checks_re);

(The variable names here seem counter-intuitive to me, but apparently when these lines are commented out, Amavisd does not invoke SpamAssassin.)

Spam level reporting
The default spam thresholds in /etc/amavis/conf.d/20-debian_defaults seem to be good, but I prefer to see the spam headers on all incoming emails. To achieve this, edit /etc/amavis/conf.d/20-debian_defaults and change $sa_tag_level_deflt = 2.0;  # add spam info headers if at, or above that level to $sa_tag_level_deflt = undef;

See Final Tweaks for a further note on this.