Burn in of new DAC

Your preamp and amp do not need to be turned on to break in the DAC, but you should play music through the DAC. 

One also has to be sure that the input to the given preamp is ’loaded’ even if it is off.

It is rare, but it can happen. One of the best way to ensure some form of decent levels of loading, is to use an older preamp with passive tape switching, where the signal can pass through the unit, if it is off, or on. then place the output of the DAC into the tape input of the preamp, and let it run with the preamp off. 99.99% of the given inputs on a preamp or whatnot, will run the signal to ’ground’ via some 47k ohm loading, but that won’t really burn anything in.

A passive switching tape input will generally load the given tape signal circuit input proper, through an approximate 1k ohm resistor, and then to the given preamp circuit and to the given second tape deck.

So we get true forms of center pin or ’hot signal’ loading, not just the standard 47k ohm connection to ground that is between all inputs and their respective grounds.

As for problems in achieving this... relay switched line inputs can sometimes be defeated when the given preamp is turned off. Which is why passive switched and loaded tape circuits are sometimes the way to go try and achieve silent burn in of things like DACS, or tuners, or what have you. (old receivers and amps, preamps, etc.. with the two tape inputs.. which were/are nearly ubiquitous for at least 40 years).