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 | evenly matched | Effectively the same output - 125W against 140W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Analogue inputs | A | Denon AVR-X4800H takes more analogue sources - 5 against 4. |
| Room to grow | evenly matched | The same expansion options on both. |
| Shelf space | depends | Denon AVR-X4800H takes up about 19% less shelf area (1781cm² against 2205cm²), and it is 97mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | evenly matched | Priced within a few per cent of each other. |
A Denon AVR-X4800H 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?
Denon AVR-X4800H has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is.
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.