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 | evenly matched | Effectively the same output - 175W against 200W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output nearly doubles (345W and 400W). |
| Analogue inputs | evenly matched | The same number of line-level inputs (1). |
| Amplifier class | depends | Naim NAP 350 is Class A/B and PS Audio PMG M400 is Class A/AB. Class A/B runs warm and is the long-established mainstream choice. Modern examples of both measure well - judge by the rest of the page. |
| Shelf space | depends | Naim NAP 350 takes up about 24% less shelf area (1371cm² against 1806cm²), and it is 89mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | Naim NAP 350 costs less - $8,499 against $8,999 at list. |
A Naim NAP 350 B PS Audio PMG M400
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.
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.