The Mullard used is New Old Stock. So that might be a contributing factor to it's lack-lustre performance. I've read somewhere that even vacuum tubes needs some time to burn-in as well, though I'm clueless as to how much burn-in time is needed for optimal sound
The initial sense of openess to the sound has been retained and it's been satisfyingly fatigue-free to listen to, in spite of the very good detail on offer. Past experience with CD has given me the impression that the "emphasis" on detail tends to be associated with listening-fatigue. I'm assuming that the "unforced" manner in which the EE DAC presents the detail has a lot to do with it's listenability
Having said that, I believe that the tube used in the DAC may make or break the sound. And rather than use an unsuitable tube, I would to stick with the solid-state output which is actually pretty good once a significant amount of burn-in has transpired...