Capacitor and resistor “burn-in” is real. My DAP’s manual even states that the sound quality will improve over the first 200 hours of normal volume, headphone-connected audio playback.
2013 Evshrug would be surprised to read this, but are you using AK’s included USB cable? Not all cables are equal... my cheap printer cable sounds awful enough that it’s a worse experience than listening to nothing, and the USB cables that came with my Blue Snowball Microphone and Chord Mojo clearly sound better than average USB cables. If you can’t find your AK cable, maybe snag a good one from Wireworld?