I have a iPod Nano 5th gen with reported specs of
 
Audio
Frequency response: 20Hz to 20,000Hz
Audio formats supported: AAC (8 to 320 Kbps), Protected AAC (from iTunes Store), HE-AAC, MP3 (8 to 320 Kbps), MP3 VBR, Audible (formats 2, 3, 4, Audible Enhanced Audio, AAX, and AAX+), Apple Lossless, AIFF, and WAV
User-configurable maximum volume limit

Headphones
Earphones
Frequency response: 20Hz to 20,000Hz
Impedance: 32 ohms
 
 
I also have HD668B headphones with the specs 
 

1. Type: Semi-open
2. Driver: 50mm, double dome, neodymium magnet
3. Sensitivity: 98 dB SPL, 1mW
4. Frequency Range: 10 to 30,000 Hz
5. Rated Impedance: 56 Ohms
6. THD: <0.3% @1KHz/1V
7. Max Input Power: 300 mW

I have a Soundblaster X-Fi Titanium Fatality I have no idea what the headphone output is on that.
 
My question is,  to get great sound  from CD quality WAV music, are my headphones good enough for the iPod, or do I need a headphone amp,  or would my money be spent wiser by getting a more expensive portable audio player like WiFiman  or Walkman player?
 
what do headphone ampliphiers actually do?  do they make the sound better, or do they make the sound louder at lower levels?
 
 

hi , 
 
im not an expert but, i have Sennheiser Momentum and I have been thinking about purchasing some amp to be able to enjoy cleaner and more controlled sound. 
try this article for the start i think it is very good good luck ! 
 
http://www.innerfidelity.com/content/ultimate-headphone-guide-articles-what-headphone-amplifier-and-why-do-i-need-one