You'll only get so far mixing with headphones. They tend to give you unnatural separation -- the left channel is exclusively in your left ear, the right channel is exclusively in your right. With speakers there's a lot more "bleed" in the stereo separation, so listening back to your headphone mix on speakers is always a bit of an eye-opener. I don't know what your budget is, but if you're looking for a mix that sounds good through speakers, you really need to use monitor speakers during the mixing process.