Sorry, yes I meant hiss. Basically, all devices have an amp. Internal amps in sources like an iPod aren't bad actually unless the phones are too sensitive. The headphones themselves don't offer a lot of resistance to the signal. So you get to enjoy everything at a magnified level, such as hiss. However, external amps offer better signal to noise ratios, meaning that the background noise the hiss, is much lower than the signal of the music. Plug in your IEM's to something like an iPod and without music, crank the volume to max, at what point can you hear hiss? with my two IEM's I can hear hiss nearly straight away. With an external amp (solid state not using tubes) I get nearly no hiss until I'm well up the volume dial. If for only this reason, an external amp is good idea, provided one doesn't mind walking around with one or being stationay with one etc.
Other advantages: all headphones need a combination of proper current and power. The more sensitive the headphone the less power they need to get to a decent volume, but they may require more current to adequately drive them. IEM's are unique in that they don't require all that much current or power, but they do tend to enjoy more current which internal opamps can't provide usually. One digital audio player that excelled with current was the Rio Karma but it is no longer available new save for auction sites. Though the iPod doesn't do too badly at all, to get the most out of such a device (because it has a nice neutral/linear sound) is to get an external amp which can provide additional current which will get a person snappier harder hitting deeper bass, better highs and sparklier mids. With IEM's this will be subtle because, again, they are already so efficient that small amounts of current and power are required to get them moving.
For me then, I use an amp to get a quieter sound experience. I presently use a Fixup Supermacro V3 but hope to try a Headamp AE-1 soon.
