In the end it comes down to you. Hey something like 50% of the enjoyment in this hobby is psychological. Many double blind and ABX tests have proven we can't reliably hear differences between cheap electronics and expensive ones. The single biggest deficiency in the music chain is the headphone. To me this means that knowing you're listening to a hand built tube amp with those specially hand selected non chinese tubes you spent weeks researching, is infinitely more pleasurable than the actual nearly inaudible differences it actually makes to the sound.
Pleasures pleasure though right?
Anyway I can't speak for others. But my own personal experience with a couple of different amps, as well as my time before headphone amps entered my life I can share this.
Fiio E9, no difference in sound quality at all, only volume. Soundstage, dynamics, all the same.
Fiio E7, no difference in sound quality at all, even barely any volume difference. Soundstage, dynamics, all the same
Soundblaster X-fi, no difference in sound quality at all. Can drive 250ohm DT880's well enough for loud music, a little too quiet for quiet music
Audioengine D1 USB DAC, slightly brighter sound with a little less compression in midrange, IE more dynamic. Can drive most headphones well on loud material, may be lacking on non efficient headphones and quiet source material.
I could keep going on and on, I have several professional recording DAC's, a music studio with various mixing consoles, headphone amps, distribution amps, etc.
The point I'm trying to make is, it's a GOOD thing that amps don't change the sound of the source material. To my thinking, the job of an amp is to be absolutely transparent. I don't WANT it to introduce anything to the signal that wasn't already there. And for the most part, most amps do a stand up job of doing this. The main differences between them are in how loud they can get. Audible differences tend to be from the op/amp chosen and the effect is far more subtle than an equalizer. If you like X type of sound, you don't need to spend money on several different amps for different op/amps, just make subtle EQ profiles to make one sound like the other.
Look up Carver amplifiers if you want to get a sense of what i mean. He was famous (infamous) early in his career for being able to take 20,000 dollar amplifiers, figure out their sound signatures, and build amps that cost a few hundred dollars that precisely mimiced that sound.
If you do want to have an amp, just to have an amp, and I can't say I blame you because I've been there, then the FiiO amps are a good start. Inexpensive, good quality, and you can send it back if you hate it.
I stand by the E9 desktop amp for the simple reason that it has 1w output power which is more than enough to drive any headphone I might ever purchase. (electrostatics use their own amps). IT was inexpensive at $80 and most importantly it doesn't introduce any coloration or noise into the signal. It just accurately amplifies the input which is the only job I ask of an amplifier. Make it louder, don't introduce any other signals, and faithfully reproduce the input.
If you go the portable dac/amp route, avoid the E7, it doesn't have enough gain to actually amplify anything. The E17, and E7k fixed this problem by including an adjustable input gain.
Good luck and be sure to come back and reply to this thread if you do buy an amp and share your thoughts on it having gone through the process. Was it everything you hoped for, etc.
Edited by Kodhifi - 2/15/13 at 7:32pm