Audio Systems: headphone amp

Question
Hi I do habe iMac and Alesis M1 Active 520 USB speakers attached to the computer via USB. And I have AKG K271 MKII headphones for which I need a headphone amp. because when I hook the headphones to the speaker's headphone port the sound is too quiet. What kind of amp do I need and how to connect all of these together. I work with a Ableton Live 8 software for producing/making music. I do have a external Native Instruments Audio DJ 8 sound card but it's not connected anywhere, will the sound card be helpful?

Answer
I have good luck with this amplifier, its not only and amplifier but
an enhance, the signal its digital and the sound quality its very noticeable

http://www.ebay.com/itm/SMSL-SAD-25-Digital-Amplifier-USB-DAC-Headphone-Amp-SMSL

good luck