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 | B | Rotel Diamond RA-6000 has more power - 235W against 180W into 8 ohms. That is about 1.2dB, which is a small step in loudness but real headroom on peaks. |
| Grip on hard loads | B | Rotel Diamond RA-6000 holds up better into a 4-ohm load: its output increases modestly to 350W, where the other barely increases. That matters with demanding speakers far more than the 8-ohm figure does. |
| Noise floor | B | Rotel Diamond RA-6000 is the quieter design on paper - 103dB against 95dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Streaming | A | Only NAD C 399 BluOS-D has AirPlay 2 and BluOS. |
| Analogue inputs | B | Rotel Diamond RA-6000 takes more analogue sources - 4 against 2. |
| Use with a TV | depends | Only NAD C 399 BluOS-D has HDMI eARC, so the television can drive it directly and one remote controls the volume. If the system is for music only, this changes nothing. |
| Room to grow | evenly matched | The same expansion options on both. |
| Amplifier class | depends | NAD C 399 BluOS-D is Class D and Rotel Diamond RA-6000 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 | A | NAD C 399 BluOS-D costs less - $3,448 against $4,499 at list. |
A NAD C 399 BluOS-D B Rotel Diamond RA-6000
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?
Rotel Diamond RA-6000 leads across most of the measured areas for a fairly modest premium, so it is the easier pick if the budget stretches. NAD C 399 BluOS-D is the one to take if it does not. If your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - Rotel Diamond RA-6000 is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating. If this sits under a television, NAD C 399 BluOS-D 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.