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 | evenly matched | Effectively the same output - 50W against 45W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Noise floor | evenly matched | Much the same signal-to-noise ratio (105dB vs 105dB). |
| Built-in DAC | A | Both decode on board, but only Denon PMA-900HNE handles DSD. |
| Streaming | A | Only Denon PMA-900HNE has AirPlay 2, Roon and HEOS. |
| Analogue inputs | B | Denon PMA-600NE takes more analogue sources - 4 against 3. |
| Room to grow | evenly matched | The same expansion options on both. |
| Shelf space | depends | Denon PMA-600NE takes up about 18% less shelf area (1405cm² against 1710cm²), and it is 70mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | Denon PMA-600NE costs less - $499 against $949 at list. |
| Watts per dollar | B | Denon PMA-600NE is the cheaper watt - about $11 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 Denon PMA-900HNE B Denon PMA-600NE
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?
Neither runs away with it - they trade advantages. Let the "depends" rows and the speakers you already own settle it. Think about the whole system: the roughly $450 saved by taking the cheaper one would buy better speakers or a subwoofer, which does more for the sound than the difference between two competent amplifiers.
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.