Headfonics review of the X18
https://headfonics.com/gustard-x18-dac-review/
"The Gustard X18 DAC offers an improved dynamic range performance, better pre-amp output power, and a smoother timbre with a much more agreeable treble overtone than the X16. It is still relatively transparent for me but with just a lick of smooth natural warmth to make it a nice easy DAC to pair with fussy revealing solid-state amplifiers. "