I'm not a big fan of tube based DACs.
I miss to see their purpose other than add coloration and distortion to the output stage.
IMHO a DAC's duty is to decode the sound and output it at the most neutral and faithful tone. Then you can alter it with a tube buffer or, like myself, with a tube amp to get a sweeter sound, closer to your needs.
Personal preferences methinks.
Maybe after all the mods my unit will sound equal or maybe better than the Costantine, who knows ;)
