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 | Onkyo Icon A-50 has more power - 110W against 76W into 8 ohms. That is about 1.6dB, which is a small step in loudness but real headroom on peaks. |
| Grip on hard loads | A | Musical Fidelity M2si holds up better into a 4-ohm load: its output nearly doubles to 137W, where the other increases strongly. That matters with demanding speakers far more than the 8-ohm figure does. |
| Noise floor | B | Onkyo Icon A-50 is the quieter design on paper - 104dB against 96dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | B | Onkyo Icon A-50 has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Use with a TV | depends | Only Onkyo Icon A-50 has HDMI, so the television can drive it directly and one remote controls the volume. If the system is for music only, this changes nothing. |
| Headphones | depends | Only Onkyo Icon A-50 has a headphone output. Worth something if you listen late, nothing if you never do. |
| Price | A | Musical Fidelity M2si costs less - $1,299 against $1,600 at list. |
A Musical Fidelity M2si B Onkyo Icon A-50
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: Onkyo Icon A-50 measures better, Musical Fidelity M2si costs about 19% less. With most speakers in most rooms that gap is smaller in practice than it looks here. If your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - Musical Fidelity M2si is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating. If this sits under a television, Onkyo Icon A-50 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.