With an ultra cheap cable, you don't know whether it's constructed properly or not. By going for something with a brand you have the assurance of reliable build and recourse to usually a fairly decent warranty. Higher-end cables are total BS in terms of audio - even at the Zu level - unless they're specifically engineered to alter the sound in some way by engineering the cable to add electrical properties. Cables like this are few and far between and for the most part (including the several thousand dollars worth of cable I own) sound the same if you're honest with yourself. I ended up repeatedly buying ultra-high-end cables firstly because I can and secondly for the assured construction quality as well as visually balance out my hi-fi rig. Garden hose diameter cables = teh pwnage visuals to round off a killer looking hi-fi rig. I think it's stupid but I do it all the same because it looks good. Same reason why I've kept buying Macs this year I think
AAC is better than MP3. It's a superior codec which means you can enjoy higher quality at equivalent bitrates.
But the level of general audio improvement from any system if you devise a cleaner way to deliver the source material to it even if it is compressed, or if you take the entire system upmarket, will sound better irrespective of the compressed signal. True, you're not maxing it out with a compressed signal but what you get is
better. And if you can feed it optically through a better DAC stage than the Mac has (which is not stellar), this counts as well - and a decent receiver is likely to be thus equipped.
Assuming the receiver is decent and has optical inputs, Airtunes fed to the receiver optically would be more flexible in terms of where you can put the computer, and it's also delivering a better signal to the receiver. Alternatively a decent-quality external soundcard + analog connection would also be a good solution. You could also consider siting the Mac elsewhere and using something like a
Squeezebox.
