Headphone Amp Question

    I've never owned a headphone amplifier before, and I'm a little confused about how they work.
   I just added a new preamp to my system, which is a significant improvement over the old one, except that it doesn't have a headphone amp.
   I also am using all of it's inputs  and do not have a spare for an amp.  there is no digital out from the preamp, but two other sources in the system. have digital outputs.
   I would like to be able to listen to my phones without having to disconnect one of the sources.  If I get an amp that will accept a digital input from one of the other sources, can I listen without having the preamp in the system?