One is a streaming all-in-one and the other is an amplifier. Judge them on how many boxes you want and what you already own, not on the spec sheet alone.
| Area | Better | Why |
|---|
| Power | B | NAD C 399 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. |
| Room size | depends | We rate NAD Masters Series M10 V2 the better fit for desk or near-field listening, a small room and a mid-size room, and NAD C 399 for a very large room. Match the amplifier to the room you actually have, not the one you want. |
| 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 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 goes higher - 384kHz against 192kHz. Very few recordings exist above 192kHz, so treat this as headroom, not as sound quality. |
| Streaming | A | Only NAD Masters Series M10 V2 has AirPlay 2, Roon and BluOS. |
| Analogue inputs | evenly matched | The same number of line-level inputs (2). |
| Phono stage | depends | NAD C 399 has a built-in phono stage (MM) and NAD Masters Series M10 V2 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. |
| Headphones | depends | Only NAD C 399 has a headphone output. Worth something if you listen late, nothing if you never do. |
| Shelf space | depends | NAD Masters Series M10 V2 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 | B | NAD C 399 costs less - $1,999 against $2,999 at list. |
| Watts per dollar | B | NAD C 399 is the cheaper watt - about $11 per watt against $30. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A NAD Masters Series M10 V2 B NAD C 399
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 is the easier recommendation: it leads on most of the measured areas and costs less. Take NAD Masters Series M10 V2 if one of the "depends" rows above describes your system. 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 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 poorly for near-field use for exactly that reason. If you play records, NAD C 399 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.