I, honestly, don't know what happens here. I dunno if you're delegating pulseaudio to jack or viseversa. Anyway, I use this method to enable my <a href="http://us.focusrite.com/usb-audio-interfaces/scarlett-18i20">Focusrite Scarlett 18i20</a> in Fedora 20. I hope it helps out:
#!/usr/bin/env bash pulseaudio -k pulseaudio -k pulseaudio -k pactl load-module module-jack-source; pactl load-module module-jack-sink
After that (in GNOME 3), just go to Settings > Sound > Output and choose "Jack sink" as the default device. It should now work as your main audio output device.
Let me know, with your comments, of any success stories or feedback you might provide.