MacBook doesn't have a DAC in it but it has a pretty good sound card.
Macs have really nice sound software. The SPDIF card (might of spelt it wrong) needs to be manually set on a mac.
The E17 is really solid. It's something like $135-$170. Pair it with a FiiO L7 to get clearer sound. A DAC is only good as the cable - make sure the cable is clean, also.
You could just save your money and not buy an amp, but you know.
Ok, would the E17 be a big upgrade over the built in sound card of the MacBook Air?