You can check if shairtunes was installed correctly by looking in the list of plugins on the Squeezebox Server’s Interface. If it is listed there with a tick, it was installed. Please make sure you removed the tick for autostart for Shairport in the Audio Player tab, otherwise Shairport will interfere with your playback and can cause problems. If you use shairtunes, all instances of Shairport need to be off. Also make sure you reboot the system after installing shairtunes or saving the option to turn off Shairport and its autostart. Then, your setup should work.