Both are NAD, from different ranges - the engineering approach is shared and the differences are mostly in power, connectivity and price.
| Area | Better | Why |
|---|
| Power | B | NAD C 399 BluOS-D has more power - 180W against 100W into 8 ohms. That is about 2.6dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output barely increases (100W and 180W). |
| Noise floor | B | NAD C 399 BluOS-D is the quieter design on paper - 95dB against 90dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | B | Both decode on board. NAD C 399 BluOS-D goes higher - 384kHz against 192kHz. Very few recordings exist above 192kHz, so treat this as headroom, not as sound quality. |
| Streaming | evenly matched | The same streaming platforms and services on both. |
| Room to grow | evenly matched | The same expansion options on both. |
| Headphones | depends | Only NAD C 399 BluOS-D has a headphone output. Worth something if you listen late, nothing if you never do. |
| Shelf space | depends | NAD M10 V3 takes up about 67% less shelf area (606cm² against 1818cm²), and it is 140mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | evenly matched | Priced within a few per cent of each other. |
| Watts per dollar | B | NAD C 399 BluOS-D is the cheaper watt - about $14 per watt against $25. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A NAD M10 V3 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, 100 watts is already more than most people ever use, and the extra 80 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.
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.