@La!$ The D50s requires 5v @ 1A according to what I could see on Topping's site.
The Raspberry Pi 3B (and 3B+) will put out a maximum of 1.2A
shared between all 4 USB ports and the RPi consumes 400mA-500mA itself.
A 2.5A 5.1v PSU should be adequate to power the RPi and the D50s, but don't expect to put much else on the RPi, like little TFT screens and such.
I'd be more inclined to just buy a cheap 5v 1A+ charger block thing. Put the USB>Barrel connector on that and power the D50s externally, and just run the other USB cable from RPi>D50s. Leaves more room for the RPi to operate as designed, and you should have enough current to drive a little screen, keyboard mouse or other little things.