Purely my own experience here, so I hope it doesn't come accross as an invalidation attempt. Everyone's experience is different, so take it for what it's worth.
Originally Posted by Spiritchaser
I'm a self proclaimed audiophile, and really haven't had much to complain about with the Pre3. There are many factors that can play into sound degredation, so it's tough to pinpoint what the issues could be. However, as someone who routinely listens to mobile music via an iPod, Pre+, iPad2, TouchPad, and Pre3...I can say that in my own experience, the Pre3 is up to snuff.
I use Music Player Remix, and have two sets of headphones I rotate based on where & how I'm listening. One pair are the Klipsch S5i headphones & the other pair are the Shure SE215's. Neither are the top of the line, but both create a nice mobile option with very good 'seals'. I have not noticed any of what you described with the Pre3. Most of the time I hear complaints about sound quality, it's due to a poor pair of headphones, a faulty wire (frayed or split beneath the shielding), or a preference of deep bass vs. a more crisp overall sound with low bass.
I'm a little stumped b/c you can go from device to device & hear a significantly negative difference, however some devices will sound better on a lower quality set of phones & some devices truly require premium phones to shine. It is possible even if you do have a premium set of phones, your set of phones may be a pair that don't add much in the way of the bass range? If that was the case & the iDevice has a higher internal bass setting, that could be your issue. I really don't know.
Just wanted to offer insight from my own experience. I don't share the same audio defficiency, so something sounds off to me. Happy to help with other ideas, but I'll wait to see what else you can find out on your own. Keep us posted.