I picked up this little FiiO headphone amplifier a few months back and have been very happy with the results. I have a pair of Sennheiser Headphones that just aren’t powered properly with my iPhone/iPod.
This little device gives my music that added umpf, adding more treble and deeper, richer bass. The device, which is about the size of an iPod Shuffle is charged via USB and so far a single charge lasts about 10-12 hours.
One word of caution, the FiiO can seriously add to the volume of your headphones, so don’t crank it up. Probably not a good gift for the teens, unless they have some serious restraint…
Purchase on Amazon
Amazon.com: FiiO E5 Headphone Amplifier: Electronics.