As a tube warms up the DC voltage seeps through the coupling caps. That is the 100uF caps in the middle of the chassis. More expensive equipment have timers and a relay to short the outputs for the first 10 seconds.
In the Bottlehead world we turn things on in a given sequence. I turn on my Eros, wait 10s, turn on my FP 2, wait 10s and then turn on my Paramours. This eliminated turn on thumps.
In the case of a self contained "system" like the Crack you just don't plug in your headsets till the thump has passed. It is a small cost for getting the quality of sound you get at an unusually low price.