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 - 72W against 60W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| 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 Primare I15 MM 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 | evenly matched | The same number of line-level inputs (5). |
| Room to grow | evenly matched | The same expansion options on both. |
| Headphones | depends | Only Rega Elex Mk4 has a headphone output. Worth something if you listen late, nothing if you never do. |
| Amplifier class | depends | Rega Elex Mk4 is Class A/B and Primare I15 MM is Class D. Class A/B runs warm and is the long-established mainstream choice; Class D runs cool and draws almost nothing at idle. Modern examples of both measure well - judge by the rest of the page. |
| Shelf space | depends | Primare I15 MM takes up about 18% less shelf area (1200cm² against 1469cm²). |
| Price | A | Rega Elex Mk4 costs less - $1,895 against $1,995 at list. |
| Watts per dollar | A | Rega Elex Mk4 is the cheaper watt - about $26 per watt against $33. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Rega Elex Mk4 B Primare I15 MM
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?
Rega Elex Mk4 has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. If you already own a DAC or a streamer, the built-in one is money you are paying twice for, and Primare I15 MM 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.