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 | Marantz Cinema 30 has more power - 140W against 110W into 8 ohms. That is about 1dB, which is a small step in loudness but real headroom on peaks. |
| Speaker load rating | A | Marantz Cinema 30 is rated down to 4 ohms against 8, so it is the safer partner for a low-impedance speaker. |
| Analogue inputs | evenly matched | The same number of line-level inputs (6). |
| Phono stage | depends | Marantz Cinema 30 has a built-in phono stage and ARCAM AVR21 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. |
| Room to grow | evenly matched | The same expansion options on both. |
| Price | evenly matched | Priced within a few per cent of each other. |
A Marantz Cinema 30 B ARCAM AVR21
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?
Marantz Cinema 30 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 play records, Marantz Cinema 30 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.