As i understand it, which is a bit foggy at best, you set up both http and https access for the Subsonic instance. Then if you enable SSL in the control panel, you still use the http address to reach SS, but it then hands off the request to the SSL connection. I don't use the SSL myself, but that's what I remember from when I tested it a while back. I always recommend not using port 80 or port 443 in Subsonic as there is just too much potential for conflicts. I've used 8081 and 441, 8080 and 442, 8084 and 444 successfully.
So, assuming you already have
http://servername.subsonic.org up and running on port 4040. Everything is working fine and your ready to set up SSL. Just chose the SSL port you're going to use, let's say 440, then set up the port forwar in the router, just like you would for 4040 ( I always recommend doing it manually) then open the SS control panel and check the "Enable https port" box and enter the correct port number that you opened on the router, in this example, 440. Save the settings. I always stop the service before editing the settings, then restart after the changes have been saved.
In theory, when you go to
http://servername.subsonic.org, Subsonic will automatically initialize the https connection.