The way that the Subsonic redirect works is that, when you start the Subsonic server up, it registers your external IP address, port number, and context path with the Subsonic.org servers. When you request your custom domain name, it responds with a "302 - Temporariliy Moved" message that includes your real address in the format of
http://<YourIPAddress>:<YourPort>/<YourContextPath>
When I put your custom domain name into my web browser (chrome) it attempted to redirect to your external IP address and port and the browser gave up after 30 seconds or so with "Oops! Google Chrome could not connect to <Your IP Address>:<Your Port>"
This tells me that the redirect is working but the traffic is not passing through your firewall.
I am running the same firewall, but have loaded TomatoUSB Firmware, so your screen will not look the same as mine, but here is my port forward page:

I am running Subsonic on ports 8080 and 8443 as you can see.
Is the port forwarding pointing to the correct internal IP address of the XP Machine?
Glenn