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 | Sony STR-DH590 has more power - 90W against 70W into 8 ohms. That is about 1.1dB, which is a small step in loudness but real headroom on peaks. |
| Analogue inputs | A | Sony STR-DH590 takes more analogue sources - 4 against 2. |
| Room to grow | evenly matched | The same expansion options on both. |
| Price | B | Yamaha RX300A costs less - $400 against $498 at list. |
A Sony STR-DH590 B Yamaha RX300A
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: Sony STR-DH590 measures better, Yamaha RX300A costs about 20% less. With most speakers in most rooms that gap is smaller in practice than it looks here.
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.