Here they state the HP chip.
https://www.blog.wiimhome.com/post/unveiling-wiim-ultra-the-digital-hub-for-your-music
TI TPA6120A2 data sheet below.
https://www.ti.com/lit/ds/symlink/tpa6120a2.pdf
Answering myself, reading Figure 6, if they implemented the chip with full 15VDC, it is 600 mW into...