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 | Sony ES STR-AZ1000ES has more power - 70W against 50W into 8 ohms. That is about 1.5dB, which is a small step in loudness but real headroom on peaks. |
| Speaker load rating | B | MartinLogan USR51 is rated down to 4 ohms against 6, so it is the safer partner for a low-impedance speaker. |
| Analogue inputs | A | Sony ES STR-AZ1000ES takes more analogue sources - 3 against 1. |
| Room to grow | evenly matched | The same expansion options on both. |
| Built-in sources | depends | Sony ES STR-AZ1000ES 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 | MartinLogan USR51 costs less - $1,300 against $1,398 at list. |
| Watts per dollar | A | Sony ES STR-AZ1000ES is the cheaper watt - about $20 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 Sony ES STR-AZ1000ES B MartinLogan USR51
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?
The usual trade-off: Sony ES STR-AZ1000ES measures better, MartinLogan USR51 costs about 7% less. With most speakers in most rooms that gap is smaller in practice than it looks here.
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.