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 | evenly matched | Effectively the same output - 100W against 85W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Streaming | A | Only WiiM Amp Ultra has Chromecast and Roon. |
| Phono stage | depends | Yamaha A-S501 has a built-in phono stage (MM) and WiiM Amp Ultra 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. |
| Headphones | depends | Only Yamaha A-S501 has a headphone output. Worth something if you listen late, nothing if you never do. |
| Amplifier class | depends | WiiM Amp Ultra 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 Ultra takes up about 69% less shelf area (522cm² against 1710cm²), and it is 134mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | evenly matched | Priced within a few per cent of each other. |
A WiiM Amp Ultra 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 Ultra 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.
What a spec sheet cannot show: how either sounds with your speakers, how the interface feels 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.