Will an Audio-gd NFB-15 (3.5W @ 25 ohms) be too powerful for the TH-x00?

Do any of you know if using optical on the NFB 15 bypasses the NFB's DAC and uses the soundcard/onboard?

Just received a NFB-15 (2014). My HE-500's are ten hours away, so I only have my ATH-M50s to play with. As a budding audio enthusiast, my initial impression is very positive. The packaging was well done, build quality seems very good, and my ATH-M50s have never been more articulate and...