No hiss from the either, more clarity and spacing from the Sony, the MBA sounds all right, the Sony very good. I just compared a couple of tracks on my 11' MBA and my Sony NWZ-1060 (Fujiya Miyagi Sixteen shades of blue, Radiohead Staircase, m4a AAC) same tracks, same ATH-M50 headphones. But be aware there is no longer a combination optical out, although of course you are free to use a USB DAC which would no doubt provide benefits. So in conclusion, most owners of older Apple laptops should be very pleased with the upgraded direct line out in the newer models. But unfortunately, I guess to cut costs, there is no combination optical/analogue output. I've also done a little audio work on it (using BFD with an electronic drumkit), and the sound is excellent - even very subtle transients and acoustics well rendered, in my opinion. With the correct headphones it is capable of thunderous bass and sub frequencies.

it totally killed the soundstage and the clarity was ruined. I've tried 'improving' the sound using a Fiio e3, and the result was awful. Used with the newer 'EarPods' this results in a modest, but refined sound, with an acceptable soundstage, good clarity, and no noticeable bias for any particular frequencies (highly subjective, I know).

In comparison, my newer Macbook Air (2011), argueably one of the finest laptops ever made, has a standard line out which is pretty damn good and a clearly noticeable improvement. However, it did have a saving grace: the 3.5mm jack was a combination SPDIF optical out, and therefore could be connected very easily to an external DAC, resulting in an excellent sound quality. My old Macbook Pro (2008) had pretty terrible line out: there was clearly audible noise, the overall signal level was quite low (as you'd expect). Please bear in mind I'm no expert, and unable to do an A-B comparison. I can do a very short assessment of the sound quality, based on my previous model which was a Macbook Pro (2008). I stumbled on this thread looking to find out what DAC my Macbook Air (2011) has.

Macbook pro digital audio out no sound