Why do you have port 80 in use? Are you intentionally running a webserver on your machine?
If you don't think you're running a webserver on your computer then you should look into why port 80 is in use.
You can use any port you like really as long as it doesn't conflict with a port on your system. Port 4040 seems to be the default choice for subsonic.
I would avoid trying to use a port that is traditionally used for another service see
http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers