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 80W into 8 ohms. That is about 1.4dB, which is a small step in loudness but real headroom on peaks. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output increases strongly (120W and 180W). |
| Noise floor | evenly matched | Much the same signal-to-noise ratio (106dB vs 104dB). |
| 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 | B | Only Onkyo Icon A-50 has AirPlay 2, Chromecast and Roon. |
| Room to grow | evenly matched | The same expansion options on both. |
| Price | A | Arcam A15+ costs less - $1,500 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 Arcam A15+ 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. Arcam A15+ 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.