These are not the same size of amplifier: NAD C 399 BluOS-D has roughly 4.5x the power of Naim Uniti Atom. That is about 6.5dB, so it is a question of room size and how hard your speakers are to drive, not of one being the better amplifier.
| Area | Better | Why |
|---|
| Power | B | NAD C 399 BluOS-D has more power - 180W against 40W into 8 ohms. That is about 6.5dB - a different class of amplifier rather than a bigger version of the same one, and far more than most rooms will ever ask for. |
| Room size | depends | We rate Naim Uniti Atom the better fit for desk or near-field listening, a small room and a mid-size room, and NAD C 399 BluOS-D for a large room and a very large room. Match the amplifier to the room you actually have, not the one you want. |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Streaming | evenly matched | Only Naim Uniti Atom has Chromecast; only NAD C 399 BluOS-D has AirPlay 2 and BluOS. Neither list is longer in a way that matters - check the one service you actually pay for. |
| Analogue inputs | B | NAD C 399 BluOS-D takes more analogue sources - 2 against 1. |
| Phono stage | depends | NAD C 399 BluOS-D has a built-in phono stage (MM) and Naim Uniti Atom does not. Decisive if you own a turntable, completely irrelevant if you do not - and an outboard phono stage is easy to add later either way. |
| Room to grow | evenly matched | The same expansion options on both. |
| Amplifier class | depends | Naim Uniti Atom is Class A/B and NAD C 399 BluOS-D is Class D. Class A/B runs warm and is the long-established mainstream choice; Class D runs cool and draws almost nothing at idle. Modern examples of both measure well - judge by the rest of the page. |
| Shelf space | depends | Naim Uniti Atom takes up about 57% less shelf area (781cm² against 1818cm²), and it is 102mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | NAD C 399 BluOS-D costs less - $3,448 against $3,799 at list. |
| Watts per dollar | B | NAD C 399 BluOS-D is the cheaper watt - about $19 per watt against $95. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Naim Uniti Atom B NAD C 399 BluOS-D
depends neither is better in the abstract - it turns on your room, your system or how you will use them. Only specs published for both amplifiers are judged; anything missing on either side is left out rather than guessed.
Which should you choose?
NAD C 399 BluOS-D has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. Start with the room. In a large or open-plan room, or with speakers that are hard to drive - low sensitivity, or an impedance that dips - NAD C 399 BluOS-D has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 40 watts is already more than most people ever use, and the extra 140 watts would be headroom you pay for and never hear - we rate NAD C 399 BluOS-D poorly for near-field use for exactly that reason. If you play records, NAD C 399 BluOS-D saves you buying a phono stage; if you do not, ignore that row entirely.
What a spec sheet cannot show: how either sounds with your speakers, how the controls feel day to day, and what the app is like to live with - which is where most all-in-one amplifiers are actually won or lost. Power figures are the manufacturers' own and are not measured to a common standard, so treat small differences as noise. If you can hear both, that will tell you more than any row above.