As I said, the mainstreamness of something is decided by how much it is listened to on last.fm. For instance, Radiohead is the most popular band on last.fm therefore in the last.fm world they're the most mainstream.
My top 5 banRAB are The Fall , Bowie , Stones , Clash & The Cure.
As I own over 200 albums just by those 5 banRAB it was pretty obvious to me I was going to be pretty mainstream.
Exactly. Having the Porcupine Tree, Beatles, RHCP, Radiohead and David Bowie in my top 15 basically destroys any chance of being non-mainstream. Looking at my top 50 banRAB isn't enough to represent my music taste, let alone my top 15.
One of my less mainstream banRAB I like is Miners of Moria with 4 listeners.