Low frequency test tones seem to be a fast and dirty way of burning in phones, especially ones that you consider to be bass-shy.
e.g. some people (er, one?
) had reported fantastic results burning in a pair of AKG K501 with a THX bass test CD.
If you go that route, be sure not to turn the volume up too loud. (we can't hear low-freq tones all that well anyway so even if you turn the volume knob up you can't hear a loud bass tone. If you do hear a loud sound it's likely to be just distortion...)