If the apple usb-c dongle dac had more voltage output, I'd have skipped buying the topping D50S to feed my THX 789. If you're happy with an upgrade, great, but the difference is probably just voltage as everyone has pointed out.
Don't believe anything we're all saying? Well, benchmark your ears to find out how sensitive you are to THD. I'm good for about -36dB THD on a good day using my etymotics(ER4XR) with my D50S+THX789 stack, but I got -30dB today using my computer speakers(Razer nomo with build in DAC). You'll find that the -109dB THD of the Khadas tone board to be
FAR above what you can discern. Now, there is a possibility that you have a playback/driver issue, or a filter issue, or a samplerate conversion issue, but you'd likely be repeating the same mistake with the new DAC.
I hope you find some value in this.
Take the klippel test!
https://www.klippel.de/listeningtest/