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 | B | Yamaha A-S801 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. |
| Noise floor | evenly matched | Much the same signal-to-noise ratio (100dB vs 99dB). |
| Built-in DAC | B | Both decode on board. Yamaha A-S801 goes higher - 384kHz against 192kHz. Very few recordings exist above 192kHz, so treat this as headroom, not as sound quality. |
| Streaming | A | Only Onkyo Muse Y-40 has AirPlay 2 and Chromecast. |
| Room to grow | B | Only Yamaha A-S801 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 Yamaha A-S801 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 1792cm²), and it is 63mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | Yamaha A-S801 costs less - $900 against $1,000 at list. |
| Watts per dollar | B | Yamaha A-S801 is the cheaper watt - about $9 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 Yamaha A-S801
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?
Yamaha A-S801 is the easier recommendation: it leads on most of the measured areas and costs less. Take Onkyo Muse Y-40 if one of the "depends" rows above describes your system.
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.