The N22 from AudioEngine is a stereo power amplifier and headphone amp in one.  It's designed to connect to your Mac or PC and pick the audio up significantly.  It's a 22 watt per channel Class A/B amp that runs cool and features a THD rating of under 0.02 percent at all power settings.

AudioEngine's N22 includes a variable line output so you can attach a subwoofer, and boasts a Burr-Brown/TI headphone amplifier designed to work with all headphones and earphones. 

You can pick up AudioEngine's N22 amplifier for a price of $199.