These are not the same size of amplifier: Musical Fidelity M6si500 has roughly 4x the power of Rega Aethos. That is about 6dB, so it is a question of room size and how hard your speakers are to drive, not of one being the better amplifier.
| Area | Better | Why |
|---|
| Power | A | Musical Fidelity M6si500 has more power - 500W against 125W into 8 ohms. That is about 6dB - a different class of amplifier rather than a bigger version of the same one, and far more than most rooms will ever ask for. |
| Price | evenly matched | Priced within a few per cent of each other. |
| Watts per dollar | A | Musical Fidelity M6si500 is the cheaper watt - about $11 per watt against $43. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Musical Fidelity M6si500 B Rega Aethos
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 M6si500 has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. Start with the room. In a large or open-plan room, or with speakers that are hard to drive - low sensitivity, or an impedance that dips - Musical Fidelity M6si500 has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 125 watts is already more than most people ever use, and the extra 375 watts would be headroom you pay for and never hear - we rate Musical Fidelity M6si500 poorly for near-field use for exactly that reason.
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.