Rane, OZ and Symetrics all make reasonably priced decent headphone amps - particularly if you can pick one up used or on e-bay. I believe OZ makes 4 and 6 user models, Symetrics makes 4 user model and Rane makes a 6 user model. Last I looked, the Rane provided the most control, but also was the most expensive of the group. A used one might run $180 or so on e-bay (I paid about that for one last year). A 4 user Symetrics older model would go for about $50 on e-bay (have one of those too) and an OZ 4 user model for about $100 (yup, got one of those also). Given the choice, I'd spring for the Rane but OZ has worked well for me (it's in my home rack while the Rane is in a coffehouse/studio rack). Others have mentioned Sampson here as an inexpensive choice, but I've never seen or used one.
Yeah I'm an attorney, but everyone needs a day job.