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 MODEL M1 has more power - 100W against 75W into 8 ohms. That is about 1.2dB, which is a small step in loudness but real headroom on peaks. |
| Grip on hard loads | A | Onkyo Muse Y-40 holds up better into a 4-ohm load: its output nearly doubles to 150W, where the other increases modestly. That matters with demanding speakers far more than the 8-ohm figure does. |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Streaming | evenly matched | Only Onkyo Muse Y-40 has Chromecast; only Marantz MODEL M1 has Roon and HEOS. Neither list is longer in a way that matters - check the one service you actually pay for. |
| Phono stage | depends | Onkyo Muse Y-40 has a built-in phono stage and Marantz MODEL M1 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. |
| Room to grow | B | Only Marantz MODEL M1 offers a subwoofer output - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Shelf space | depends | Marantz MODEL M1 takes up about 55% less shelf area (552cm² against 1225cm²), and it is 96mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | evenly matched | Priced within a few per cent of each other. |
| Watts per dollar | B | Marantz MODEL M1 is the cheaper watt - about $10 per watt against $13. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Onkyo Muse Y-40 B Marantz MODEL M1
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 MODEL M1 has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. If your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - Onkyo Muse Y-40 is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating. If you play records, Onkyo Muse Y-40 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.