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-S980H has more power - 90W against 50W into 8 ohms. That is about 2.6dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Analogue inputs | B | Denon AVR-S980H takes more analogue sources - 4 against 1. |
| Phono stage | depends | Denon AVR-S980H has a built-in phono stage and MartinLogan USR51 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-S980H 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. |
| Price | B | Denon AVR-S980H costs less - $949 against $1,300 at list. |
| Watts per dollar | B | Denon AVR-S980H 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 MartinLogan USR51 B Denon AVR-S980H
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-S980H is the easier recommendation: it leads on most of the measured areas and costs less. Take MartinLogan USR51 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-S980H 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 40 watts would be headroom you pay for and never hear. If you play records, Denon AVR-S980H 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.