Wiim Mini into DX5 ii question.

I'm running the Wiim to the DX5 using an optical cable, listening to hi res streams from Amazon Music Unlimited, which is controlled by the Topping Home app on my android phone. I notice the volume can be controlled by the phone which leads me to think I'm not taking full advantage of the 24 bit stream. Am I going about this wrong? I'm listening with HD600's through the 4 pin XLR. It seems like I'm getting better sound streaming Internet radio in flac Bluetooth to the DX5 than I get using optical. I find the hi res from Amazon has way boosted treble for my tastes.