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-S501 has more power - 85W against 60W into 8 ohms. That is about 1.5dB, which is a small step in loudness but real headroom on peaks. |
| Room size | depends | We rate WiiM Amp the better fit for desk or near-field listening and a small room, and Yamaha A-S501 for a mid-size room, a large room and a very large room. Match the amplifier to the room you actually have, not the one you want. |
| Noise floor | evenly matched | Much the same signal-to-noise ratio (98dB vs 99dB). |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Streaming | A | Only WiiM Amp has AirPlay 2, Chromecast and Roon. |
| Phono stage | depends | Yamaha A-S501 has a built-in phono stage (MM) and WiiM Amp does not. Decisive if you own a turntable, completely irrelevant if you do not - and an outboard phono stage is easy to add later either way. |
| Room to grow | evenly matched | The same expansion options on both. |
| Amplifier class | depends | WiiM Amp is Class D and Yamaha A-S501 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 | WiiM Amp takes up about 75% less shelf area (420cm² against 1710cm²), and it is 173mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | WiiM Amp costs less - $299 against $550 at list. |
| Watts per dollar | A | WiiM Amp is the cheaper watt - about $5 per watt against $6. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A WiiM Amp B Yamaha A-S501
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?
WiiM Amp has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. If you play records, Yamaha A-S501 saves you buying a phono stage; if you do not, ignore that row entirely. Think about the whole system: the roughly $251 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.