As stated, the receiver is not in imminent danger. This is especially true for casual listening. At louder listing levels, you'll get more fatigue due to distortion artifacts from low bit rate MP3s than anything else. That said, the iPod Touch is a really good player (as verified
here).
If you you equalize the low and high frequencies, the mid band will clear up, and with less high frequency artifacts, the fatigue might be less.
If the Apple Lossless format is used, I don't see any issues with fatigue either.
Call me old fashioned, but, I use WAV files or disc media for any critical listening...