These are not the same size of amplifier: Onkyo Muse Y-50 has roughly 2.1x the power of Rotel A12 MKII. That is about 3.2dB, so it is a question of room size and how hard your speakers are to drive, not of one being the better amplifier.
| Area | Better | Why |
|---|
| Power | A | Onkyo Muse Y-50 has more power - 125W against 60W into 8 ohms. That is about 3.2dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Noise floor | evenly matched | Much the same signal-to-noise ratio (100dB vs 100dB). |
| Built-in DAC | B | 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 | A | Only Onkyo Muse Y-50 has AirPlay 2 and Chromecast. |
| Room to grow | evenly matched | The same expansion options on both. |
| Amplifier class | depends | Onkyo Muse Y-50 is Class D and Rotel A12 MKII 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-50 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 | Rotel A12 MKII costs less - $1,100 against $1,500 at list. |
| Watts per dollar | A | Onkyo Muse Y-50 is the cheaper watt - about $12 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 Onkyo Muse Y-50 B Rotel A12 MKII
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?
The usual trade-off: Onkyo Muse Y-50 measures better, Rotel A12 MKII costs about 27% less. With most speakers in most rooms that gap is smaller in practice than it looks here. Start with the room. In a large or open-plan room, or with speakers that are hard to drive - low sensitivity, or an impedance that dips - Onkyo Muse Y-50 has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 60 watts is already more than most people ever use, and the extra 65 watts would be headroom you pay for and never hear. Think about the whole system: the roughly $400 saved by taking the cheaper one would buy better speakers or a subwoofer, which does more for the sound than the difference between two competent amplifiers.
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.