Unfortunately, if content is coming from an ad-server, that is the referring domain.
While html headers can be configured to pass through original referrers, you are not the only client of DoubleClick and they wouldn't make a change to their system that would help you and potentially hurt all other clients (who actually want and need to know that the traffic was coming from the ad server).
You could use campaign tracking (cid or UTMs) on all the ads links (hopefully with code that would programmatically / dynamically populate source domain information), then use this information to attribute the traffic to the "specified" source.