The lightening to 30 pin adaptor has a DAC built in. IIRC it is a Wolfson DAC, comparable in quality to the built-in DAC in the 5, 5C, and 5S audio codecs (all different, actually, and all made by Cirrus Logic). But, in any case, Apple wants control over these sorts of things, not just the adaptor but the cables as well. It's standard Apple stuff, and can be seen in any number of different lights. Personally, I don't think there's any question that the Lightening connector is technically superior over the old 30 pin or micro-USB, and is especially future-proof for various reasons (among them being dynamically reassignable pins). When flash memory is fast enough in the iPhones and iPods, I think you'll see Lightening-to-USB-3.0 and Lightening-to-Thunderbolt cables.
In any case, going from HO to an external amp will mean double amping your signal, which is never ideal, to say the least. On the other hand, you can get digital out direct to an outboard DAC/AMP combo. Or you can use the adaptor and get analog line out on the usual pins of the 30 pin port.