I have the Nagra tube dac, with external power supply, in the context of a Nagra separates system. Despite (or maybe because of) the price, it's not a flashy dac -- no audiophile fireworks, no jazz-hands "look at me" kind of presentation. Natural, fluid sound, dynamically engaging, sounds like music to me -- I can listen spellbound for hours.
Is the MSB Reference DAC better, for your system and taste? I can't answer that. If you can home demo the Nagra tube dac, great -- if not, go with the MSB, which you've already tried and loved -- don't worry about comparisons you can't make!