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 45W into 8 ohms. That is about 2.8dB, so it plays usefully louder before it runs out and has more in hand 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 Denon PMA-600NE 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 | B | Denon PMA-600NE is the quieter design on paper - 105dB against 99dB. 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 4. |
| Room to grow | evenly matched | The same expansion options on both. |
| Shelf space | depends | Denon PMA-600NE takes up about 18% less shelf area (1405cm² against 1710cm²), and it is 70mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | Denon PMA-600NE costs less - $499 against $550 at list. |
| Watts per dollar | A | Yamaha A-S501 is the cheaper watt - about $6 per watt against $11. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Yamaha A-S501 B Denon PMA-600NE
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, Denon PMA-600NE costs about 9% 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 - Yamaha A-S501 has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 45 watts is already more than most people ever use, and the extra 40 watts would be headroom you pay for and never hear - we rate Yamaha A-S501 poorly for near-field use for exactly that reason.
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.