Well, the NDAC/555PS is currently Naim's reference DAC setup, so it is hardly soprising you enjoy it
If you went that path, then you will enjoy it more and more as you add to the capability of the NACs and NAPs... you are in Naim seperates territory... it is a somewhat different world to the integrated territory as perhaps you are now discovering.. but clearly the integrateds like the 272 provide a great compromise .. but ultimately it's just not got to be the same performance of the seperates.
Much of the Naim approach to top SQ appears to be about space and decoupling around componentry, careful ground planing/shielding and through hole componentry to avoid EM coupling, physical modulation and ground plane modulation... just look at the innards of the 282/252/552/NDAC/NDS for example..but clearly that requires expense and 'real estate'. The alternative requires squeezing a lot using more surface mount and/or more dense componentry into a small box such as the 272 and the Uniti ranges with very careful decoupling and spacing. They do incredibly well in the circumstances.. but ultimately are going to be compromised in terms of SQ by this component density.
its horses for courses..