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 | A | 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 | A | Onkyo Icon A-50 holds up better into a 4-ohm load: its output increases strongly to 180W, where the other increases modestly. That matters with demanding speakers far more than the 8-ohm figure does. |
| Noise floor | evenly matched | Much the same signal-to-noise ratio (104dB vs 106dB). |
| Built-in DAC | A | Onkyo Icon A-50 has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Streaming | A | Only Onkyo Icon A-50 has AirPlay 2, Chromecast and Roon. |
| Room to grow | evenly matched | The same expansion options on both. |
| Price | B | Marantz PM8006 costs less - $1,499 against $1,600 at list. |
| Watts per dollar | A | Onkyo Icon A-50 is the cheaper watt - about $15 per watt against $21. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Onkyo Icon A-50 B Marantz PM8006
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. Marantz PM8006 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 - Onkyo Icon A-50 is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating. If you already own a DAC or a streamer, the built-in one is money you are paying twice for, and Marantz PM8006 becomes the more rational buy.
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.