A valve amplifier against a solid-state one. They are not really rivals: they differ in character, in what they will drive and in what owning them involves, so start there rather than with the watts.
| Area | Better | Why |
|---|
| Power | B | NAD C 399 BluOS-D has more power - 180W against 100W into 8 ohms. That is about 2.6dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Room size | depends | We rate Rogue Audio Cronus Magnum III the better fit for desk or near-field listening, a small room and a mid-size room, and NAD C 399 BluOS-D for 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 | NAD C 399 BluOS-D has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Analogue inputs | A | Rogue Audio Cronus Magnum III 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. |
| Amplifier type | depends | Rogue Audio Cronus Magnum III is Tube and NAD C 399 BluOS-D is Solid State. Valve designs are usually warmer and softer at the limits and need their tubes replaced eventually; solid state is the lower-maintenance choice. This is a taste and ownership decision, not a quality one. |
| Price | evenly matched | Priced within a few per cent of each other. |
| Watts per dollar | B | NAD C 399 BluOS-D is the cheaper watt - about $19 per watt against $35. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Rogue Audio Cronus Magnum III 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?
NAD C 399 BluOS-D has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. Start with the room. In a large or open-plan room, or with speakers that are hard to drive - low sensitivity, or an impedance that dips - NAD C 399 BluOS-D has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 100 watts is already more than most people ever use, and the extra 80 watts would be headroom you pay for and never hear - we rate NAD C 399 BluOS-D poorly for near-field use for exactly that reason. 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.