One is a streaming all-in-one and the other is an amplifier. Judge them on how many boxes you want and what you already own, not on the spec sheet alone.
| Area | Better | Why |
|---|
| Power | A | NAD C 379 BluOS-D has more power - 80W against 60W into 8 ohms. That is about 1.2dB, which is a small step in loudness but real headroom on peaks. |
| Noise floor | B | Rotel A12 MKII is the quieter design on paper - 100dB against 95dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | A | Both decode on board, but only NAD C 379 BluOS-D handles MQA. |
| Streaming | A | Only NAD C 379 BluOS-D has AirPlay 2, Roon and BluOS. |
| Room to grow | evenly matched | The same expansion options on both. |
| Amplifier class | depends | NAD C 379 BluOS-D is Class D and Rotel A12 MKII is Class A/B. Class D runs cool and draws almost nothing at idle; 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. |
| Price | B | Rotel A12 MKII costs less - $1,100 against $1,499 at list. |
A NAD C 379 BluOS-D B Rotel A12 MKII
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 379 BluOS-D measures better, Rotel A12 MKII costs about 27% less. With most speakers in most rooms that gap is smaller in practice than it looks here. Think about the whole system: the roughly $399 saved by taking the cheaper one would buy better speakers or a subwoofer, which does more for the sound than the difference between two competent amplifiers.
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.