The issue was the fact that I am using rvm, which causes issues with sass paths (thanks to this SO answer).
To solve this, I generated an rvm gemset with:
# will create 'php_sass' and 'php_compass' executables in $GEM_HOME
rvm wrapper ruby-1.9.3-p194 php sass compass
And added the following to my config.yml file:
assetic:
debug: %kernel.debug%
use_controller: true
bundles: [ ]
filters:
cssrewrite: ~
scss:
sass: /Users/Nick/.rvm/bin/php_sass
apply_to: "\.scss$"
SCSS files now compile just fine. Compass users will need to point assetic to the gemset for compass