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 | evenly matched | Effectively the same output - 150W against 180W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Room size | depends | We rate NAD M32 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 (150W and 180W). |
| Noise floor | A | NAD M32 is the quieter design on paper - 124dB against 95dB. 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. |
| Analogue inputs | A | NAD M32 takes more analogue sources - 4 against 2. |
| Use with a TV | depends | Only NAD C 399 has HDMI eARC, so the television can drive it directly and one remote controls the volume. If the system is for music only, this changes nothing. |
| Price | B | NAD C 399 costs less - $2,899 against $3,299 at list. |
| Watts per dollar | B | NAD C 399 is the cheaper watt - about $16 per watt against $22. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A NAD M32 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?
Neither runs away with it - they trade advantages. Let the "depends" rows and the speakers you already own settle it. If this sits under a television, NAD C 399 is the one to buy - HDMI ARC means the TV remote controls the volume and there is no second box in the way.
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.