Seegee:
In the same episode Anya casually mentions that she and Xander have tried spanking. She doesn’t say who spanked who, but Anya strikes me as a top.
Agreed, especially since she's a former 'vengeance demon' whose function was to punish (in worse ways than via spanking) men who mistreat women. Xander doesn't strike me as being all that assertive either.
In response to Willow's reference to Buffy's "brand-spanking-new boyfriend" (Riley), Anya tells her that "We've enjoyed spanking,"
Willow has no 'significant other' at this point in time, so she pouts that "Everybody's getting spanked except me."
Hopefully she eventually ends up taking a few bare-bottom-up trips across Tara's lap... --C.K.