These are not the same size of amplifier: Rega Elex Mk4 has roughly 2.4x the power of Rega io. That is about 3.8dB, 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 | Rega Elex Mk4 has more power - 72W against 30W into 8 ohms. That is about 3.8dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Room size | depends | We rate Rega Elex Mk4 the better fit for a mid-size room, a large room and a very large room, and Rega io for desk or near-field listening and a small room. Match the amplifier to the room you actually have, not the one you want. |
| Built-in DAC | A | Rega Elex Mk4 has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Analogue inputs | A | Rega Elex Mk4 takes more analogue sources - 5 against 2. |
| Room to grow | A | Only Rega Elex Mk4 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 64% less shelf area (522cm² against 1469cm²), and it is 50mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | Rega io costs less - $725 against $1,895 at list. |
A Rega Elex Mk4 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?
The usual trade-off: Rega Elex Mk4 measures better, Rega io costs about 62% less. With most speakers in most rooms that gap is smaller in practice than it looks here. 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 - Rega Elex Mk4 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 42 watts would be headroom you pay for and never hear - we rate Rega Elex Mk4 poorly for near-field use for exactly that reason. 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.