These are not the same size of amplifier: Yamaha A-S301 has roughly 2x the power of Onkyo A-9110. That is about 3dB, 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 | B | Yamaha A-S301 has more power - 60W against 30W into 8 ohms. That is about 3dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Noise floor | A | Onkyo A-9110 is the quieter design on paper - 103dB against 99dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | B | Yamaha A-S301 has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Room to grow | evenly matched | The same expansion options on both. |
| Shelf space | depends | Onkyo A-9110 takes up about 16% less shelf area (1434cm² against 1701cm²), and it is 61mm 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 | Yamaha A-S301 is the cheaper watt - about $6 per watt against $12. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Onkyo A-9110 B Yamaha A-S301
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-S301 has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. 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 - Yamaha A-S301 has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 30 watts is already more than most people ever use, and the extra 30 watts would be headroom you pay for and never hear. If you already own a DAC or a streamer, the built-in one is money you are paying twice for, and Onkyo A-9110 becomes the more rational buy.
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.