the Japanese despite well... being in Japnese is hard to understant but gives more technical info.
apparently the Optoplay has a built-in amp
here are the stats from the Jap site
AKM AK4353 DAC (Same as the Sonica)
DynamicRange102dB S/N 102dB
Spes from the US site
Interface: USB Spec Ver1.1, USB Audio Class Spec Ver1.0
Outputs: Analog & Digital Audio 2-channel output
Output Format: Analog Line or Headphone output, SPDIF Optical Output
Sample Rate: 32 kHz, 44.1 kHz, 48 kHz, 64 kHz, 88.2 kHz, 96 kHz
Resolution: 16 bits upto 24 bits
Analog output Level: 1.0 Vrms Maximum.
Headphone Amp power: 50 mW Maximum.
Power: USB Bus powered - 100 mA
WIN 98SE/ME/2000/XP/MAC OS X compatible