My initial problem was caused by an older version of Untangle Firewall. Since the one I have been running was quite old, I just rebuilt it with the newest one. (Love VM's!) The newest version (9.1?) has default settings for the spam filter which do not interfere at all with email hosting.
My secondary problem was the socket, as you mentioned. ClamAV was not configured AT ALL in amavisd.conf. Don't know why, perhaps when I updated not all settings were updated as well.
Thank you again for your fresh eyes and simple suggestions. They made finding my problem far more simple than it could have been.