Now, some of you might be wondering: why would I need a headphone amp? After all, don't most DAPs and PMPs have enough power at their disposal to run most headphones without amplification, and isn't it A VERY BAD THING to turn your headphones up too loud? Well, yes and no. First, depending on impedance and sensitivity, some headphones need more power than others to run at the same perceived volume level. Secondly, PMPs and DAPs vary in terms of output power, and size isn't any indication.
For instance, the Sansa Clip or Samsung YP-P3 pump out enough juice to get a big sound from most sensible pairs of headphones, but many Apple players, some Sony models and most iPod shuffle type players are a bit more restrained in their output.
Plug in some high-impedance in-ear 'phones or a pair of over-the-ear cans and many players struggle to create a decent sound. The result is that you pump up the volume levels, pushing the amplification circuitry in your player too hard. The sound compresses, and you're not really getting the full potential from either your headphones or your player. Keeping the volume levels on your player to a moderate setting and letting an outboard amplifier do the work can be a better idea.
The knock-on effect of this is that the little FiiO can be a fantastic addition to your portable audio setup, though a lot depends on what player and headphones you are using. During the last few days I've tried it out on three different players - a first-gen iPod touch, a Samsung YP-Q1 and an iRiver T7 - with a bewildering variety of headphones.