It does already have a decent built in DAC. It really depends on the quality of audio you plan on streaming and the quality of you're other components. If you want hifi quality sound youre gonna want a separate DAC imo.

Have you looked into a Chromecast Audio? Super simple/cheap solution if you want to stream via wifi. Pair it with a cheap dac (topping d30) and youre set for ~$150.

Here's the response I got from the amazon seller, it's similar to yours: "For your questions, please kindly note we have already contact with TOPPING, their technician said it's because the D50 has added a function of slowly increasing volume to prevent sudden access signals that scare people or...

I recently purchased a Topping D50 and am running across the exact same issue. Seems to work great with usb from computer, but the optical from TV occasionally drops the audio signal, and sometimes very frequently. Did you ever figure out a solution? Love this DAC and would hate to return...