SU-9 has around 1 bit of better multi-tone result, higher voltage output. X16 has ~1dB more SINAD. I would assume they are about equivalent.. buy what is cheaper or which you like the looks of X16 vs A18: the X16 is better in just about anything, objectively speaking? Unless you really want a collectible AK4499 DAC while AKM supplies still last, I'd opt for the newer X16, myself.
Thanks a lot.