Here's what I remember: the story was written as part of a kink table (challenge?) and incorporated a huge list of kinks, including the one that makes it my favorite: urethral sounds. It was about 50 chapters and had Spike as Xander's dominant.
Does anyone remember this and know where to look? Many thanks in advance!
EDIT: Thanks so much to uisgich! The story is Demagnetized by creyr.