Well I like perhaps some (or many?) have listened and enjoyed the NDAC, NDX, NDS, ND555 as well as various Chord DACs including Hugo, TT, Hugo2 and Dave. To me there is a characteristic similar sound to the Naim DACs, perhaps based on similar analogue filtering, i2S, as well as PCM1704Ks and the same AD DSP methods... although refinement and design clearly improves through the range and the ND555 has been rather resolving when I have listened with it, and moves things nicely on from the NDS, albeit I don’t find it has the weight of the NDAC/555PS which was quite something... albeit perhaps not top flight in resolution.
The Chord DACs tend to have a similar trait amongst themselves as well, and to my ears I have yet to hear a more enjoyable all round DAC than the original Hugo, although Dave takes the lead with hidef, and sample variation with the Hugo is greater than I would prefer.. I ended up buying my demo one as it sounded so much better than two other new Hugos I tried. To my ears I just find them potentially more natural sounding than PCM1704K based devices..but that is subjective, and all DACs have a choice of compromise.
So in the end I think it really falls down to sonic preference and ‘house’ style. To me some of the greatest ND555 advancements have been through design decoupling and electrical noise management from the streaming functions and display etc.