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 | Musical Fidelity M2si has more power - 76W against 55W into 8 ohms. That is about 1.4dB, which is a small step in loudness but real headroom on peaks. |
| Room size | depends | We rate Yamaha WXA-70A the better fit for desk or near-field listening and a small room, and Musical Fidelity M2si for a mid-size room, a large room and a very large room. Match the amplifier to the room you actually have, not the one you want. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output nearly doubles (105W and 137W). |
| Built-in DAC | A | Yamaha WXA-70A has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Analogue inputs | B | Musical Fidelity M2si takes more analogue sources - 5 against 1. |
| Phono stage | depends | Musical Fidelity M2si has a built-in phono stage (MM) and Yamaha WXA-70A 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. |
| Use with a TV | depends | Only Yamaha WXA-70A has HDMI eARC/ARC, so the television can drive it directly and one remote controls the volume. If the system is for music only, this changes nothing. |
| Shelf space | depends | Yamaha WXA-70A takes up about 67% less shelf area (587cm² against 1760cm²), and it is 133mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | evenly matched | Priced within a few per cent of each other. |
| Watts per dollar | B | Musical Fidelity M2si is the cheaper watt - about $17 per watt against $24. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Yamaha WXA-70A B Musical Fidelity M2si
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?
Musical Fidelity M2si has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. If you play records, Musical Fidelity M2si saves you buying a phono stage; if you do not, ignore that row entirely. If this sits under a television, Yamaha WXA-70A is the one to buy - HDMI ARC means the TV remote controls the volume and there is no second box in the way.
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.