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 | 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 PS Audio Stellar Strata MK2 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. |
| Grip on hard loads | A | PS Audio Stellar Strata MK2 holds up better into a 4-ohm load: its output increases strongly to 150W, where the other barely increases. That matters with demanding speakers far more than the 8-ohm figure does. |
| Built-in DAC | B | Both decode on board, but only NAD C 399 BluOS-D handles MQA. |
| Analogue inputs | B | NAD C 399 BluOS-D takes more analogue sources - 2 against 1. |
| 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. |
| Shelf space | depends | PS Audio Stellar Strata MK2 takes up about 19% less shelf area (1481cm² against 1818cm²), and it is 75mm shallower, which is the measurement that decides whether a rack door shuts. |
| 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 PS Audio Stellar Strata MK2 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. If your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - PS Audio Stellar Strata MK2 is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating. 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.
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.