Depends on the headphones. What specific headphones are you talking about? If the headphones are not being properly driven by your current source, then it's certainly possible that an amp will bring out more bass. On the other hand, if the headphones are very efficient and simply have a response that is light in bass, then no, adding an amp will not increase the bass. An amp adds gain to the signal and makes more power available when demanded by the signal and the load.