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 | Onkyo Muse Y-40 has more power - 75W against 60W into 8 ohms. That is about 1dB, 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 100dB). |
| Built-in DAC | A | Both decode on board. Rotel A12 MKII goes higher - 384kHz against 192kHz. Very few recordings exist above 192kHz, so treat this as headroom, not as sound quality. |
| Streaming | B | Only Onkyo Muse Y-40 has AirPlay 2 and Chromecast. |
| Room to grow | evenly matched | The same expansion options on both. |
| Amplifier class | depends | Rotel A12 MKII is Class A/B and Onkyo Muse Y-40 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 | Onkyo Muse Y-40 takes up about 25% less shelf area (1225cm² against 1633cm²), and it is 29mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | Onkyo Muse Y-40 costs less - $1,000 against $1,100 at list. |
| Watts per dollar | B | Onkyo Muse Y-40 is the cheaper watt - about $13 per watt against $18. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Rotel A12 MKII B Onkyo Muse Y-40
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?
Onkyo Muse Y-40 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.