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 - 150W against 180W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Grip on hard loads | A | Cambridge Audio Evo 150 holds up better into a 4-ohm load: its output increases strongly to 250W, where the other barely increases. That matters with demanding speakers far more than the 8-ohm figure does. |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Streaming | evenly matched | Only Cambridge Audio Evo 150 has Chromecast; only NAD C 399 BluOS-D has BluOS. Neither list is longer in a way that matters - check the one service you actually pay for. |
| Analogue inputs | evenly matched | The same number of line-level inputs (2). |
| Room to grow | evenly matched | The same expansion options on both. |
| Shelf space | depends | Cambridge Audio Evo 150 takes up about 35% less shelf area (1190cm² against 1818cm²), and it is 46mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | Cambridge Audio Evo 150 costs less - $3,249 against $3,448 at list. |
A Cambridge Audio Evo 150 B NAD C 399 BluOS-D
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?
Cambridge Audio Evo 150 has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. If your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - Cambridge Audio Evo 150 is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating.
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.