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 | evenly matched | Effectively the same output - 70W against 60W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Speaker load rating | B | JBL MA310 is rated down to 4 ohms against 6, so it is the safer partner for a low-impedance speaker. |
| Analogue inputs | evenly matched | The same number of line-level inputs (2). |
| Room to grow | evenly matched | The same expansion options on both. |
| Built-in sources | depends | Yamaha RX300A has a radio tuner built in. That is one less box if you still play discs or listen to radio, and dead weight if you do not. |
| Amplifier class | depends | Yamaha RX300A is AB and JBL MA310 is 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 | Yamaha RX300A takes up about 17% less shelf area (1396cm² against 1672cm²), and it is 65mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | Yamaha RX300A costs less - $400 against $440 at list. |
| Watts per dollar | A | Yamaha RX300A is the cheaper watt - about $6 per watt against $7. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Yamaha RX300A B JBL MA310
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?
Neither runs away with it - they trade advantages. Let the "depends" rows and the speakers you already own settle it.
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.