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 | evenly matched | Effectively the same output - 75W against 75W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output nearly doubles (150W and 150W). |
| Noise floor | A | Onkyo Muse Y-40 is the quieter design on paper - 100dB against 74dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | B | Both decode on board, but only Marantz Stereo 70s handles DSD. |
| Streaming | evenly matched | Only Onkyo Muse Y-40 has Chromecast; only Marantz Stereo 70s has Roon and HEOS. Neither list is longer in a way that matters - check the one service you actually pay for. |
| Room to grow | B | Only Marantz Stereo 70s offers a subwoofer output - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Amplifier class | depends | Onkyo Muse Y-40 is Class D and Marantz Stereo 70s is Class A/B. Class D runs cool and draws almost nothing at idle; Class A/B runs warm and is the long-established mainstream choice. Modern examples of both measure well - judge by the rest of the page. |
| Shelf space | depends | Onkyo Muse Y-40 takes up about 32% less shelf area (1225cm² against 1803cm²), and it is 59mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | evenly matched | Priced within a few per cent of each other. |
A Onkyo Muse Y-40 B Marantz Stereo 70s
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 Stereo 70s has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is.
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.