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 - 60W against 70W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Room size | depends | We rate Naim Nait 5si the better fit for desk or near-field listening and a small room, and Marantz Model 40n for a mid-size room, a large room and a very large room. Match the amplifier to the room you actually have, not the one you want. |
| Built-in DAC | B | Marantz Model 40n has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Analogue inputs | A | Naim Nait 5si takes more analogue sources - 4 against 3. |
| Phono stage | depends | Marantz Model 40n has a built-in phono stage (MM) and Naim Nait 5si 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. |
| Use with a TV | depends | Only Marantz Model 40n has HDMI ARC, so the television can drive it directly and one remote controls the volume. If the system is for music only, this changes nothing. |
| Shelf space | depends | Naim Nait 5si takes up about 39% less shelf area (1300cm² against 2126cm²), and it is 180mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | Naim Nait 5si costs less - $2,299 against $2,499 at list. |
A Naim Nait 5si B Marantz Model 40n
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 you play records, Marantz Model 40n saves you buying a phono stage; if you do not, ignore that row entirely. If this sits under a television, Marantz Model 40n 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.