I use the Musicbox Webclient (not the whole image, I hope this still fits in this category) and I edited /usr/local/lib/python2.7/dist-packages/mopidy_musicbox_webclient/static/js/streamuris.js, but the Streams-Page still shows the old defaults.
I even tried this on a computer that had never accessed it before, so it can’t be cache/cookies.
EDIT: Nevermind, I had to edit the file /usr/local/lib/python2.7/dist-packages/Mopidy_MusicBox_Webclient-2.0.0-py2.7.egg/mopidy_musicbox_webclient/static/js/streamuris.js instead.