There are 2 ways of looking at the function of an external headphone amplifier:
- To make up for a deficiency in the source (player)
- To make up for a deficiency in the headphone
If the built-in headphone amp of the player is unable to drive the headphone adequately, an external headphone amp would be helpful. Typically, this is a scenario where an MP3 player is unable to supply enough power (voltage and/or current) to properly drive the attached headphone.
Also, some MP3 players' native headphone amp don't handle an external load (i.e. headphones) well, and can result in an unsatisfactory sound signature (e.g. weak bass).
Headphones with high impedance or low sensitivity may require more "juice" than the MP3 player has, and low-impedance headphones may demand too much current from the portable player.
Attaching an external headphone amplifier, even out of the headphone jack of the portable player, potentially addresses the above issue.
Sometimes, some choose to use an amplifier to "correct" the sound signature of their headphones. For example, some may choose a "warm" sounding amp with an analytical "cold" headphone. Kinda like using the amp as an external equalizer. And some amps allow you to adjust the bass (and treble).
This is potentially problematic as what might work for one headphone might not work for another, and the whole is often greater than the sum of its parts - synergy may not always give you the results you want.
Your ER6i was designed to be used without an external headphone amplifier, straight out of the headphone jack of an iPod back in the day when it was well-known that loading an iPod's headphone jack changed it's sound signature quite a bit. The ER6i is a more sensitive, slightly bass-boosted version of the ER6.
Bassheads like me want more bass than the ER6i has, and rather than adding an external amp to the equation, I picked a different bass-rich IEM. Some would add a cheap amp like a FiiO which adds a bass boost (and noise as well) to add some fun.
The ER6i has great isolation, and a clean sound many love, and Etymotic has a very loyal fan base. I'm sure you'll love the clarity and detail of the Etymotic.
Listen to your ER6i for an extended period and take notes. Rather than jumping at adding an amplifier right away, take note of what you like and don't like. After a while, you'll have a much clearer idea of what your next upgrade should sound like (e.g. a lush midrange, comfort, better soundstage, etc). It's an individual thing.
There are many who bought an Etymotic years ago, and are still using it today. You may be one of those who choose to use the ER6i in a portable setting because of it's hard-to-beat noise isolation, then spending money on a desktop amp and full-sized headphone for home use.
