Dear all, thank you for your responses to my initial post and the interesting discussion that followed. I don't know whether my Gustard requires burn-in, but anyway I've moved from a coaxial cable to an optical cable from the Gustard to the Hugo. For all talk of 1s and 0s there is to my ears a clear difference.
The coaxial cable produces in my system a slightly hollowed-out sound which is slightly less natural to my ears, but on the other hand the soundstage is wider than ever and the timing is excellent - really engaging. This is the first time I've really experienced the famed 'PRaT' (I have no Naim components yet), although with some albums it can be a little fatiguing.
However, the problem with the cut-outs when lights and other electrical devices are switched in the vicinity of the living room has not been solved by the Gustard set-up, so I have moved to an optical connection which (as previously) avoids this problem. The music is now to my ears more natural (and of course, uninterrupted), though a little less rhythmic and with a narrower soundstage.
Unfortunately the Gustard has been acting up a bit in the last week - it has started to lose connection with the Mac Mini when not in use. Normally, whether in use or not, the Gustard displays the sample rate on its display (in my case 44.1). Now, when not in use for a while (and this 'while' can be minutes or hours), the Gustard display shows '----' which I believe indicates that it is on, but does not know the sample rate. In this state, I cannot play music from Audirvana. It can be rectified by unplugging and replugging the USB cable (either out of the Mac or into the Gustard), but I don't want to have to do this all the time.
This problem occurs with both optical and coaxial cables out and (more relevant, as the problem is probably upstream) with 2 different USB cables. I did install the latest update to Audirvana during the week, thought I cannot remember if my problem followed or preceded that update.
Has anyone else with a Gustard experienced this '----' issue?
Many thanks for any advice!