Well if its any help, i will quickly give you my findings as like you i had a NDS/555dr before i went looking for the whats next step.
Like you probably, i have been into naim for over 25 years and got the naim sound that i enjoy built in and its hard sometimes to move away from it.
DAVE DAC :- found it was great at detail but lacked weight and prat and just didn’t do it for me at all, plus i wouldn’t be surprised to see a new version out this year, with a better headphone amp and possibly the m scaler built in, so perhaps not a great time to buy one new, plus dac only its expensive and start adding m scaler, streamer and its stand to try and make it look nicer, then its suddenly got very expensive.
Added bonus can also do pre amp duties if digital only system.
ND555 :- didn’t offer a bigger difference over the nds, yes it was slightly better but certainly not worth the £10k difference in price, but i can see why people buy it as its the easy option to keep the naim sound plus the extra features its brings are nice, but again i already feel its being left behind.
DCS BARTOK :- as a one box solution at £10k i found this simply great, the new mapping sounds detailed and has nice prat, and i found it very close to the Naim ND555, infact it was hard to separate them on sound and if being totally honest i think the Bartok just has it and at its price point thats a bargin, plus you can add the clock to further enhance it and with DCS approach to future updates and upgrades then its not something you will need to be swapping any time soon and thats very nice to have in this fast changing area of hifi, plus you can have the headphone version if you so wish at extra cost.
Added bonus can also do pre amp duties if digital only system.
DCS ROSSINI :- well i think you already know about this one, right from the start it just grabbed me and has not even now let me go, its just fantastic.
It has everything, all the best bits Dave can do and chucked them into the ND555 and the result it great detail and prat. It what i wished i had heard when i first went to demo the ND555.
But just when you think it can’t get any better than this, throw in the clock and it all just gets a bit better, dont ask me why as i havent a clue, but you can certainly hear the difference and i have twice sat down off an evening and played a track and thought that doesn’t sound quite right, only to realise i haven’t switched the clock on.
Plus it has the upgrades and updates capabilities that DCS offer going forward and has already had the 2.0 upgrade since its launch which made a massive difference to the way it sounds.
Plus i must just add the streamer is superb and the quality off qobuz is great and on par almost with downloads on the melco, but also mine is the player version and until the other day had not even tried it, what a mistake that was, its cd playback quality is stunning and beats streaming and anything the melco can throw into it ( at the moment )
Added bonus can also do pre amp duties if digital only system.
Never tried the premier so cant give you any views against the above, but if you are looking at say the Bartok with a future view off adding the clock, i would try and resist if you can till you can get the rossini, as the Bartok and clock are about the same price as the rossini and then add a clock to the rossini when you can.
As like i said ths rossini and clock is something you should try as i really do rate it highly.