Why so good on headphones but a mess on monitors?

One of the best pieces of advice I've had is this: Use one speaker only, and turn the master into mono. Now try to make your mix sound good. Not only do you get the benefits of balancing stuff mono, but you also minimize room interference and reflections and such, because you use a single speaker.

Even with good near field monitors (I still use Genelec 1031a's), a decent subwoofer makes a huge difference in hearing correct bass response. It also liberates the work that the near field monitors do. And still, I find it useful to confirm this with headphones now and then.