No single best amplifier here. What decides it is what you have to drive, what you need to plug in, and how much of the system you want in one box.
| Area | Better | Why |
|---|
| Power | B | NAD C 389 has more power - 130W against 76W into 8 ohms. That is about 2.3dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Room size | depends | We rate Musical Fidelity M2si the better fit for a small room, and NAD C 389 for a large room and a very large room. Match the amplifier to the room you actually have, not the one you want. |
| Grip on hard loads | A | Musical Fidelity M2si holds up better into a 4-ohm load: its output nearly doubles to 137W, where the other barely increases. That matters with demanding speakers far more than the 8-ohm figure does. |
| Noise floor | evenly matched | Much the same signal-to-noise ratio (96dB vs 95dB). |
| Built-in DAC | B | NAD C 389 has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Analogue inputs | A | Musical Fidelity M2si takes more analogue sources - 5 against 3. |
| Use with a TV | depends | Only NAD C 389 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. |
| Headphones | depends | Only NAD C 389 has a headphone output. Worth something if you listen late, nothing if you never do. |
| Amplifier class | depends | Musical Fidelity M2si is Class A/B and NAD C 389 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. |
| Price | A | Musical Fidelity M2si costs less - $1,299 against $1,999 at list. |
A Musical Fidelity M2si B NAD C 389
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 your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - Musical Fidelity M2si is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating. 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 389 has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 76 watts is already more than most people ever use, and the extra 54 watts would be headroom you pay for and never hear - we rate NAD C 389 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.