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 | B | 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. |
| Built-in DAC | A | Both decode on board. MISSION 778X goes higher - 384kHz against 192kHz. Very few recordings exist above 192kHz, so treat this as headroom, not as sound quality. |
| Price | evenly matched | Priced within a few per cent of each other. |
| Watts per dollar | B | Yamaha A-S501 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 MISSION 778X 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?
Yamaha A-S501 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-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 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.