We do it through Polk and our CRM administers it for us. Polk does a double Opt-in for email append and no one has more data than Polk. I have heard that AB is switching from Polk to Authenticom because supposedly they got better returns from Authenticom. However, we are staying with Polk. Some shady things with the Autobase switch:
A) Dominion who owns Autobase also owns Authenticom - so I don't know how much credit I give to their claims of better results. I think it's more of a case of cost for Autobase and a matter of getting business to Authenticom for Dominion.
B) I have had issues with Authenticom in the past, as have many others I know and EVEN Autobase before they were owned by Dominion.