Question About My Amplifier?

Hey I just recently bought a Vox VT100 amplifier and it has always sounded pretty good, but also dark and a little unresponsive with way too much low end. I recently plugged in headphones in the headphone jack and, WOW! The amp could PERFECTLY imitate Eddie Van Halen, Slash, Angus Young, you name it. And it sounded 500x better. What is causing this? Is there anything I can do to get this amp to play like this without headphones?