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 | B | Denon AVR-X1800H has more power - 80W against 50W into 8 ohms. That is about 2dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Analogue inputs | evenly matched | The same number of line-level inputs (3). |
| Room to grow | A | Only Marantz Cinema 70s offers preamp outputs - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Shelf space | depends | Denon AVR-X1800H takes up about 18% less shelf area (1471cm² against 1801cm²), and it is 69mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | Denon AVR-X1800H costs less - $849 against $1,300 at list. |
| Watts per dollar | B | Denon AVR-X1800H is the cheaper watt - about $11 per watt against $26. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Marantz Cinema 70s B Denon AVR-X1800H
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?
Denon AVR-X1800H 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 - Denon AVR-X1800H has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 50 watts is already more than most people ever use, and the extra 30 watts would be headroom you pay for and never hear. Think about the whole system: the roughly $451 saved by taking the cheaper one would buy better speakers or a subwoofer, which does more for the sound than the difference between two competent amplifiers.
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.