Through some silly oversight, I didn’t have Perreaux listed under the hardware links. Well, that’s fixed now. Perreaux is a New Zealand brand of electronics, especially well respected for its 
amplifiers. Incredibly it has been around since 1974 and, I’m fairly certain, far exceeds any Australian amplifier brand in sales reach. It has now added to its range the SHX-1 headphone amplifier.
Why a headphone amplifier? While the great majority of audiophiles prefer to use speakers, some actually prefer headphones. There are advantages to these. For one thing, no speaker setup can match a fine pair of headphones for delivering detail. Regardless of how good the speakers are. The reason is simple: the listening room. There will always be reflections that will cause sounds to mask each other. This doesn’t happen with headphones.
Me? I only use headphones as a last resort, or to clarify something that I’m hearing from a recording. Headphones are the ultimate in unrealistic sound. The stereo stage cannot be resolved with headphones (unless you are using some pretty nifty DSP processing which merely aims to replicate the affects of a room on sound, which rather misses the point). Instead, you get a spread of intruments and vocals through your brain.
I’m sure, though, that Perreaux will have implemented a headphone amplifier as well as anyone could. One thing to note on portable audio gear (Walkmans, MP3 players and so forth) is that very few of them do bass well. Virtually all of them roll off the bass, typically to between -7dB and -20dB at 20 hertz. It comes with the territory. The low voltage power sources just can’t deliver the current necessary to sustain high bass levels into low impedance headphones. Things are better if your headphones offer a hundred or more ohms impedance, but most these days are around 32 ohms. This is highly unlikely to be a problem with the SHX-1, nor indeed for most headphone outputs on equipment driven by mains power.