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 | Denon DRA-900H has more power - 100W against 80W into 8 ohms. That is about 1dB, which is a small step in loudness but real headroom on peaks. |
| Built-in DAC | A | Both decode on board, but only Denon DRA-900H handles DSD. |
| Streaming | A | Only Denon DRA-900H has AirPlay 2 and HEOS. |
| Analogue inputs | B | Arcam A15 takes more analogue sources - 3 against 2. |
| Use with a TV | depends | Only Denon DRA-900H has HDMI ARC, so the television can drive it directly and one remote controls the volume. If the system is for music only, this changes nothing. |
| Room to grow | evenly matched | The same expansion options on both. |
| Price | A | Denon DRA-900H costs less - $799 against $999 at list. |
| Watts per dollar | A | Denon DRA-900H is the cheaper watt - about $8 per watt against $12. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Denon DRA-900H B Arcam A15
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?
Denon DRA-900H is the easier recommendation: it leads on most of the measured areas and costs less. Take Arcam A15 if one of the "depends" rows above describes your system. If this sits under a television, Denon DRA-900H 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.