Sarcher30 raises some interesting points. A preamp having very low output impedance, and the ability to drive low impedance loads without significant frequency response issues or increases in distortion, should work well into many headphones (especially those that combine high impedance with good voltage sensitivity). But those are a lot of constraints, and if the preamp and phones were not chosen very carefully it would be easy to end up with poor sonic results.
Regards,
-- Al
Is there a non active way to drop the impedance down enough to work with headphones?A suitably chosen step-down transformer would lower output impedance, while also reducing gain. I'm not aware of any transformers that are specifically designed to interface between preamps and headphones, though, and again I would imagine it would require careful selection of all of the components to achieve a good match. It might also cost more than many dedicated headphone amps.
Regards,
-- Al