These are not the same size of amplifier: Rega Aethos has roughly 2.5x the power of Denon PMA-900HNE. That is about 4dB, so it is a question of room size and how hard your speakers are to drive, not of one being the better amplifier.
| Area | Better | Why |
|---|
| Power | A | Rega Aethos has more power - 125W against 50W into 8 ohms. That is about 4dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Built-in DAC | B | Denon PMA-900HNE has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Phono stage | depends | Denon PMA-900HNE has a built-in phono stage (MM,MC) and Rega Aethos does not. Decisive if you own a turntable, completely irrelevant if you do not - and an outboard phono stage is easy to add later either way. |
| Price | B | Denon PMA-900HNE costs less - $949 against $5,395 at list. |
| Watts per dollar | B | Denon PMA-900HNE is the cheaper watt - about $19 per watt against $43. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Rega Aethos B Denon PMA-900HNE
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 PMA-900HNE has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. Start with the room. In a large or open-plan room, or with speakers that are hard to drive - low sensitivity, or an impedance that dips - Rega Aethos has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 50 watts is already more than most people ever use, and the extra 75 watts would be headroom you pay for and never hear. If you play records, Denon PMA-900HNE saves you buying a phono stage; if you do not, ignore that row entirely.
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.