An amp takes a signal and amplifies it. Others can explain this better than me, but here we go. Let's say your DAP can drive you headphones, but you are near max volume to get them to your desired level. The amp will then take that signal and amplify it and give you more head room in volume. It's technically not supposed to color the signal, and just amplify it. However, people hook up the amp and go wow, but the truth is that it's just louder and nothing more. If you were to volume match, there would be no difference. We tend to perceive louder as better this or that. Amps have many benefits obviously. They can help you drive high impedance headphones and IEMs. Some offer bass boost to color the sound. They can also extend the battery life of your DAP because most line out ports bypass the internal amp causing the DAP to use less power. Maybe someone will come along with a more detailed answer, but that's the basics on amps as explained to me on head-fi. ETA: Brooko noted he heard no difference between balanced and SE when the volume was matched, so that tells me there is no increase in soundstage when using balanced vs. SE. Balanced has increased power to drive higher impedance headphones, and from my research, that's it. Like I said, we perceive louder as better, but in reality the only difference is output power.