Need a portable amp, budget $150

Are the headphones not satisfactory straight out of the iPod? The reason I ask is that their impedances aren't particularly low (removing any major advantage from the lower output impedance of a portable amp) and they're relatively high sensitivity (compared with other audiophile headphones) Unless they simply don't go loud enough for your tastes, there seems little point in an amp.

EDIT: Checking Headroom's graphs of impedance vs frequency for your headphones, output impedance mismatch wouldn't have any real effect on the frequency response anyway - the impedance graph for both headphones is virtually completely flat over the audible range. 

Impedance is a measurement of a headphone's opposition to current. It takes into account capacitance and inductance, unlike a normal DC resistance measurement and hence varies with frequency.

Headphones come with a variety of impedances, ranging from 16 to 600 ohms. You can find plenty about headphone impedance and how it affects things with Google. What I was interested in here was the output impedance of the iPod and the impedance of your headphones over a range of frequencies.

When the output impedance is too high, we get a significant voltage drop at the source. This voltage drop varies depending on the headphone impedance - consider the voltage "split" across the two resistances.

The problem is that many headphones have impedances that vary significantly over the audible frequency range. This means that the voltage drop also varies with frequency - hence leading to potential changes in the sound of your headphones, as different audio frequencies will become quieter or louder depending on the impedance at that frequency.

However, your headphones' impedance appears pretty constant over the frequency range, so any potential advantages from getting a headphone amplifier with a lower output impedance than the headphone-out of your iPod is minimal.

As to it sounding better, unless your headphones are too quiet with your iPod no need to purchase an amplifier.