Mobile amps don't distort audio quality but rather give you the ability to turn up the volume louder. In other words, the signal (normally voltage) is amplified. Mobile amps will often let people hear encoding artifacts easier due to the increased signal. That is why many people use higher bitrate music whether it be AAC or mp3. That being said, it is all up to the user whether or not they will hear a difference between a 320kbps AAC file, Apple lossless file (don't use AIFF), or even a 128kbps VBR AAC file.
The best thing that you can do is plug your portable amp into your computer's audio output and then conduct a blind ABX test using foobar2000. Make sure you use the headphones that you plan on normally using. That will be replicate the experience. Also keep in mind that portable environments have many outside influences that mask frequencies even with amps. Quiet portable environments are the key moments when amps really shine. Otherwise, the built-in drivers on iPods provide sufficient results unless your main goal is to "pump the volume."
So conduct a blind ABX test and determine if you can hear the differences between a lossless file and various lossy formats/settings. Lossy formats have come a long way now and that extremely old notion of "you must use 320kbps to achieve the best quality" (not that you were saying that) no longer holds true. There used to be a time when listening to audio with portable amps where one needed 192kbps+ music but that just isn't the case anymore as the Lame mp3 encoder (version 3.98) can produce transparent quality at -V 3 which is about 170kbps VBR no matter what is driving the music (portable amp, built-in driver, studio amp, car amp, etc.).
64GB iPhone 5 | 64GB iPad mini | AppleTV 2 (2012) | AppleTV 2 (2010) | 2012 15" MacBook Pro, 1TB SSHD, 16GB DDR3 1600 MHz, OS X 10.8.3 Mountain Lion | Apple Lossless | iTunes AAC 192kbps VBR | iTunes 11.0.2| Library size = 1.04TB | Legacy iPods: 3G 40GB, 4G 40GB, 5G 60GB, 160GB iPod classic (2009)