I'm kind of old but my idea of an amp was something that takes a line level signal, amplifies it and then sends it out, usually to speakers.  But things in the headphone world are different.  Now, it seems that the headphone out on a DAC/Amp frequently packs less punch than the line out (my experience is with USB powered devices). So, it's easy for me to think that a DAC/Amp is only a DAC with an attenuator thrown in.  Obviously there must be more to it than that.
 
I'm sure this has been discussed before but I don't know how to go about searching for it.  If anyone could explain or provide links I would be grateful.  Thanks.