Naim have advocated from the very beginning that to achieve the highest performance, transformers should be kept separate from the most delicate parts of the amplification chain (this is for a variety of factors that can adversely impair performance). It’s why the pre-amps have no transformers inside. It’s also why the top sources don’t either (or at slightly lower level, can be upgraded to bypass the internal supply). Even the power amps eventually separate the amp from the transformer with appreciable gains - just compare a NAP250DR against a NAP300DR; they are essentially the same amp, but the 300 separates the amp from the transformer and adds some extra cooling to take best advantage of the increased real estate.
Mechanical factors are just as important as electronic ones (mechanical engineers have a large area within R&D), which is why Naim go to such lengths to create decoupled and suspended parts, even going as far as creating mass loaded suspended PCBs, decoupled connectors both for audio and electrical input.