I just ran into this about an hour ago when I installed AmahiTunes. I had to edit /etc/mt-daapd.conf while logged into a command-line session on the host itself, as opposed to through the http://hda
Once you've done that, and saved the changes, you can either restart mt-daapd with "service mt-daapd restart" or reboot the host through the http://hda
interface. I tried restarting AmahiTunes from its Status page, but I found that to be rather hit or miss for me. Sometimes it would restart, other times, it would stop, but not start back up, requiring me to log into the host through SSH and do the restart with the service command as mentioned above. So, now I just do it all through command-line.
(I know this was an old question, but since I just had the same problem, I figured I'd post what I did to get around it, in case somebody else comes along with the same issue.)