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 | A | Arcam A5 has more power - 50W against 30W into 8 ohms. That is about 2.2dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Room size | depends | We rate Arcam A5 the better fit for a very large room, and Rega io for desk or near-field listening. Match the amplifier to the room you actually have, not the one you want. |
| Built-in DAC | A | Arcam A5 has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Analogue inputs | A | Arcam A5 takes more analogue sources - 3 against 2. |
| Room to grow | A | Only Arcam A5 offers preamp outputs - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Shelf space | depends | Rega io takes up about 67% less shelf area (522cm² against 1580cm²), and it is 74mm 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 | A | Arcam A5 is the cheaper watt - about $14 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 Arcam A5 B Rega io
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?
Arcam A5 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 - Arcam A5 has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 30 watts is already more than most people ever use, and the extra 20 watts would be headroom you pay for and never hear. If you already own a DAC or a streamer, the built-in one is money you are paying twice for, and Rega io becomes the more rational buy.
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.