A good pair of cans for your portable. Basics, power.

2. Cans: Slang for headphones ( urbandictionary.com)

A good pair of headphones ( my general term for earbuds, fullsize, ear-canal headphones, and other stuff essentially designed to be used in close proximity to the ears) is essential for MP3 players, other portable audio players, and even computer users who don't want to use speakers.
In fact some headphones easily rival speaker systems costing hundreds and thousands of dollars.

This post is about the power the headphones require, they are basically mini speakers. The amount of power needed and available will affect your sound quality.

Hugh Robjohns gives a nice technical explanation of impedance on his site, and notes that sticking within 8-32 ohms range is generally a good idea for portable players (http://www.practical-home-theater-guide.com/headphones.html).

So when you're at the store looking at headphones, the biggest ones are not necessarily the best pair for your portables. If you paid a higher impedance headphone with a portable the device will need to be used on higher volume to sound "okay". This will wear down your battery faster, more importantly the headphones will struggle to reproduce the sound. It's like having cheap speakers without an amplifier, turning up the volume will only do so much.

For practical portable use, choose headphones with a lower impedance design.
www.headphone.com walks you through the process with detailed reviews of more headphones than you can imagine, and finds the right fit for portables. Many earbuds, like the white ones which come with the ipod have low impedance, but a rare number do not. Do yourself a favor and upgrade from the stock ipod earbuds.

Option B: If you want to lug around a larger pair of cans (stop laughing) or use very high impedance headphones even on your computer, a headphone amplifier may be necessary. This essentially boosts the power of the signal, and products can also clean up signal noise resulting in a better listening experience. The "soundstage" or essentially all that you hear from the music will open up with the help from an amp, and some medium impedance headphones can benefit. Beware, this is a pricey route.

http://www.headphone.com/products/headphone-amps/
Would be a good starting point to look at some amps, and they make their own.

Power is just one important factor when considering headphones, but an important one.

More to come....