Although this is an annoying effect, I'm fairly certain that isn't the problem. The Grado headphones fix the problem yet do not include a hardware crossfeed. Also, I use bs2b[1] (Bauer stereophonic-to-binaural DSP) which fixes all but the worst mixes (i.e. old music such as the Beatles with each guitar mixed pure L/R).

[1] http://bs2b.sourceforge.net/