No single best amplifier here. What decides it is what you have to drive, what you need to plug in, and how much of the system you want in one box.
| Area | Better | Why |
|---|
| Power | A | 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 Yamaha A-S501 the better fit for a mid-size room, a large room and a very large room, and OSD Nero Stream XD2 for desk or near-field listening and a small room. Match the amplifier to the room you actually have, not the one you want. |
| Noise floor | A | Yamaha A-S501 is the quieter design on paper - 99dB against 95dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Analogue inputs | A | Yamaha A-S501 takes more analogue sources - 6 against 1. |
| Phono stage | depends | Yamaha A-S501 has a built-in phono stage (MM) and OSD Nero Stream XD2 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 | Yamaha A-S501 is Class A/B and OSD Nero Stream XD2 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 | OSD Nero Stream XD2 takes up about 60% less shelf area (688cm² against 1710cm²), and it is 112mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | OSD Nero Stream XD2 costs less - $400 against $550 at list. |
A Yamaha A-S501 B OSD Nero Stream XD2
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: Yamaha A-S501 measures better, OSD Nero Stream XD2 costs about 27% less. With most speakers in most rooms that gap is smaller in practice than it looks here. 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 $150 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.