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 - 76W against 80W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| 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 | Cambridge CXA81 II is the quieter design on paper - 105dB against 96dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | B | Cambridge CXA81 II has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Analogue inputs | evenly matched | The same number of line-level inputs (5). |
| Phono stage | depends | Musical Fidelity M2si has a built-in phono stage (MM) and Cambridge CXA81 II 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. |
| Headphones | depends | Only Cambridge CXA81 II has a headphone output. Worth something if you listen late, nothing if you never do. |
| Price | B | Cambridge CXA81 II costs less - $1,199 against $1,299 at list. |
A Musical Fidelity M2si B Cambridge CXA81 II
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?
Cambridge CXA81 II has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. 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 you play records, Musical Fidelity M2si saves you buying a phono stage; if you do not, ignore that row entirely.
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.