I now have 2 DACs (Mensa Plus DI/O, Scott Nixon TubeDac) and recently put the Mensa Plus back in the system after listening to the TubeDac for a few days. I had heard that the Mensa Plus can take a while to warm up but I have learned that it is as long as 4 days... if not longer! It must be all the large capacitors...
I plugged the Mensa Plus back in on Saturday, and the sound progressively got clearer, cleaner, smoother and generally more musical over the next few days, including yesterday (Wednesday). I don't feel I was splitting hairs. I listened to several familiar tracks several times each day.
So, I think that the Mensa Plus (may apply to smART and Mensa upgrades as well) should be kept plugged in for at least 4 days straight in order to make a fair comparison with another DAC. This is similar to comments made on the long warm-up time of amps like the Odyssey Stratos. By the way, the Scott Nixon TubeDac seemed to be fully warmed up in an hour or so, but keep it in mind it has very few parts.
In normal use outside of DAC comparison shootouts, the long warm up time won't be an issue - the DAC would stay plugged in 24 hours a day.
