For ND555 - prior to present Firmware update
Server Mode : on
Air Play: off
Bluetooth: off
Digital 1: on
Digital 2: off
Digital 3: on
Digital 4: on (was off before)
Qobuz: on (turn off if not using - worthwhile)
Spotify: on
Tidal - input enabled : on
Remote - Mobile Detect: off
Chromecast: on (default - not tested yet)
Some being ‘on’ sounded better - even though I don’t use the input, so I tried all and this worked best when I tried it.
…these were all very small micro-improvements that removed slight phasy blurrings and ‘bumps’.
Some settings ‘off’ were worse in that subjective ‘suck-outs’ in presentation happened.
A lot of people will not notice these effects I think, but on some revealing systems it matters and I easy hear them here.
I have not revisited these - and the updates recently have turned-on things like Chromecast and Qobuz that I don’t use and can’t recall testing on/off, so thse are to be determined ahead.
After the recent Firmware update for ND555 I tend to spend some time getting used to its altered presentation before then revisiting the settings but I will over then next few days and will post when I do.
My method is to play two or three different types of music tracks which have vocal, piano, deep bass notes providing undepinning timing and synth - I try the settings and see what makes it more interesting to listen to and enjoyable or no difference and move-on to the next - then re-visit later if I think something has got worse.
Some settings give an immediate effect of ‘better-clearer’, but at expense of musical cohesion being lost so you need to not make too quick a final decision. I recall Server Mode was one of these.
Have fun. 
DB.