Thanks for your quick responses guys.
Since the original penguin update back in April 2012, we've cleaned up our link profile immensely, improved the load speed of our site by over 150% and totally reworked & simplified our UI. Throughout all this we've provided unique, daily content.
As such it's been annoyance that we've only seen our rankings drop, but frankly we've never touched our core of fansite links. I'll be quite surprised if these are indeed the source of our problems - but at the same time delighted to have finally found the culprit.
Still, ignoring whatever decisions Google has made in its algorithms, are these fansites (and ourselves) actually doing anything wrong with their sitewide links? What is it that Google doesn't like about them? Usually the individuals who run the sites provide a lot of up-to-date content that other fans like to see, and quite often users will be interested in similar artists/bands, hence the links to "friends" or "affiliate" sites in the sidebar.
Is this a niche way of doing things that probably should have an exception from Google's calculations?
Or are they just bad, bad, bad?