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. |
| Phono stage | depends | Denon AVR-X1800H has a built-in phono stage and Anthem MRX SLM 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. |
| Built-in sources | depends | Denon AVR-X1800H has a radio tuner built in. That is one less box if you still play discs or listen to radio, and dead weight if you do not. |
| Amplifier class | depends | Anthem MRX SLM is D and Denon AVR-X1800H is AB. Class D runs cool and draws almost nothing at idle; Class A/B runs warm and is the long-established mainstream choice. Modern examples of both measure well - judge by the rest of the page. |
| Shelf space | depends | Anthem MRX SLM takes up about 37% less shelf area (923cm² against 1471cm²). |
| 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 Anthem MRX SLM 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 is the easier recommendation: it leads on most of the measured areas and costs less. Take Anthem MRX SLM if one of the "depends" rows above describes your system. 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. If you play records, Denon AVR-X1800H 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.