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 | Onkyo TX-RZ70 has more power - 140W against 100W into 8 ohms. That is about 1.5dB, which is a small step in loudness but real headroom on peaks. |
| Analogue inputs | A | Anthem MRX 540 8K takes more analogue sources - 5 against 4. |
| Phono stage | depends | Onkyo TX-RZ70 has a built-in phono stage and Anthem MRX 540 8K 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 | Onkyo TX-RZ70 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 540 8K is AB+D and Onkyo TX-RZ70 is AB. 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 540 8K takes up about 20% less shelf area (1754cm² against 2205cm²), and it is 100mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | Anthem MRX 540 8K costs less - $1,900 against $2,300 at list. |
A Anthem MRX 540 8K B Onkyo TX-RZ70
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?
Neither runs away with it - they trade advantages. Let the "depends" rows and the speakers you already own settle it. If you play records, Onkyo TX-RZ70 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.