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 | Onkyo Icon A-50 has more power - 110W against 70W into 8 ohms. That is about 2dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output increases strongly (120W and 180W). |
| Noise floor | B | Onkyo Icon A-50 is the quieter design on paper - 104dB against 90dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | B | Both decode on board. Onkyo Icon A-50 goes higher - 768kHz against 192kHz. Very few recordings exist above 192kHz, so treat this as headroom, not as sound quality. |
| Streaming | evenly matched | Only NAD C 359S has BluOS. Neither list is longer in a way that matters - check the one service you actually pay for. |
| Room to grow | evenly matched | Only NAD C 359S offers a subwoofer output; only Onkyo Icon A-50 offers preamp outputs - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Price | A | NAD C 359S costs less - $1,299 against $1,600 at list. |
| Watts per dollar | B | Onkyo Icon A-50 is the cheaper watt - about $15 per watt against $19. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A NAD C 359S B Onkyo Icon A-50
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?
Onkyo Icon A-50 leads across most of the measured areas for a fairly modest premium, so it is the easier pick if the budget stretches. NAD C 359S is the one to take if it does not.
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.