No single best amplifier here. What decides it is what you have to drive, what you need to plug in, and how much of the system you want in one box.
| Area | Better | Why |
|---|
| Power | B | Marantz PM-10 has more power - 200W against 160W into 8 ohms. That is about 1dB, which is a small step in loudness but real headroom on peaks. |
| Grip on hard loads | B | Marantz PM-10 holds up better into a 4-ohm load: its output nearly doubles to 400W, where the other increases strongly. That matters with demanding speakers far more than the 8-ohm figure does. |
| Analogue inputs | evenly matched | The same number of line-level inputs (5). |
| Phono stage | depends | Marantz PM-10 has a built-in phono stage (MM,MC) and Rega Osiris 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. |
| Headphones | depends | Only Marantz PM-10 has a headphone output. Worth something if you listen late, nothing if you never do. |
| Amplifier class | depends | Rega Osiris is Class A/B and Marantz PM-10 is Class D. Class A/B runs warm and is the long-established mainstream choice; Class D runs cool and draws almost nothing at idle. Modern examples of both measure well - judge by the rest of the page. |
| Shelf space | depends | Rega Osiris takes up about 28% less shelf area (1519cm² against 2108cm²), and it is 127mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | Marantz PM-10 costs less - $8,999 against $11,600 at list. |
| Watts per dollar | B | Marantz PM-10 is the cheaper watt - about $45 per watt against $73. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Rega Osiris B Marantz PM-10
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?
Marantz PM-10 is the easier recommendation: it leads on most of the measured areas and costs less. Take Rega Osiris if one of the "depends" rows above describes your system. If your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - Marantz PM-10 is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating. If you play records, Marantz PM-10 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.