It's because the AT's DAC/amp that I use doesn't have usb support but I do have a USB DAC and it's proven more flaky. The advantage of an optical connection is that it doesn't need drivers and so you only need to rely on the constructor being good with hardware and not on them being good on the software side of things (which most Japanese company cannot do).

In this case, I use a DAC combined with the Amp but honesty while I see the difference that an Amp brings, I don't see the difference of using a different DAC than apple's DAC (I've tried doing blind tests). So, I could see myself just using an headphone amp without using a DAC..