Considering the NAD is about $100 more than the AI-101DA I'd suggest comparing the NAD to TEAC's AI-301DA.
The AI-301DA has a more robust amplifier, and I believe a better DAC than the AI-301DA (as I understand Burr Brown's better DACs have a lower number. Someone correct me if I'm wrong.) I assume from NAD not publicizing their headphone output specs that it is at maximum 50mw, whereas both TEACs have a 100mw headphone amplifier.