The whole premise here actually has more to do with software since it’s often said that the iPhone 4s sounds a little nicer in a straight comparison. Since I haven't yet seen software on iDevices discussed almost at all anywhere on the internet, I decided to run my own test on my iPhone 4 – and I definitely found that it makes a big difference: iOS 4: clean, clear and fluid – even more so when I remove unnecessary LaunchDaemons iOS 5: sounds pretty good, but not as revealing as iOS 4 iOS 6: I couldn’t even enjoy the music anymore – too much detail was lost, very muddy sounding With the iPhone 4, you can upgrade/downgrade to any iOS version you like so long as you had saved .SHSH tickets. You cannot do this with any other iPhone that comes afterwards – so even if later iPhones have superior hardware (debatable), none can run their software as lean as the iPhone 4 – and in my experience, that clearly affects the sound you end up with. As a point of technical comparison, on reboot, the system and background processes on my iPhone 4 (with unnecessary processes removed) occupy just 25 MB of RAM with 10 background processes which is very lean indeed. But on iOS 8, the system occupies 800 MB on my iPhone 6 Plus with 140 running processes (no apps running). I actually really enjoy music on my iPhone 6 Plus too (did Apple do something to the soundstage?) and it might be my favorite device if it would run iOS 4. But in the real world, I prefer my iPhone 4 setup to the other iDevices I've listened to, even when I've used popular external DACs. My iPhone 4 setup: LOD > O2 Amp > Senn HD650 or Sony MDR-7506 with iOS 4.0.1 (unnecessary processes removed) and it sounds really nice! Very musical, almost analog if I can use that word. Pretty clear, very robust and warm sounding. You can hear the ringing of the instruments – a level of clarity that I don’t find on most setups I’ve tried. The level of emotional engagement here is much better than any other combination I’ve tried – including the Teac HA-P50, which sounded clinical and very clear, but lost a lot of the midrange. I compared this against other external DACs (Fiio E17k, Fiio E18, and the ODAC) via my my iPad 1, and iPhone 6 Plus, but none of these setups could match the presence, warmth, and drive when using the LOD on my iPhone 4 with iOS 4. (ODAC clearly not at its best on mobile devices) I may end up trying the Fostex HP-P1 – which has been very well reviewed – but otherwise, I’ve tried a lot of other mobile DACs – and it’s very hard to beat the combination I have here. I haven't heard the iPod 5.5 or iMod, but I'm reluctant to try that since there's not an overwhelming consensus that the iPod 5.5 is better - some ppl complain about its bass rolloff for example. Hopefully, this is helpful to others especially if you're looking for a nice mobile DAP that doesn't cost too much - although it does require some technical ability. If anybody else happens to have an iPhone 4 with a saved .shsh ticket for iOS 4.0.1, I'd be curious to hear what sort of results you get.