Siblings in NAD's Classic range, so they share a design and a house sound. It comes down to power, features 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. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output barely increases (150W and 180W). |
| Noise floor | evenly matched | Much the same signal-to-noise ratio (95dB vs 95dB). |
| Built-in DAC | B | Both decode on board, but only NAD C 399 BluOS-D handles MQA. |
| Streaming | evenly matched | The same streaming platforms and services on both. |
| Analogue inputs | evenly matched | The same number of line-level inputs (2). |
| Room to grow | evenly matched | The same expansion options on both. |
| Price | A | NAD C 388 BluOS-2i costs less - $2,249 against $2,599 at list. |
A NAD C 388 BluOS-2i 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?
The usual trade-off: NAD C 399 BluOS-D measures better, NAD C 388 BluOS-2i costs about 13% less. With most speakers in most rooms that gap is smaller in practice than it looks here.
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.