Different price classes - Denon AVR-X6800H costs roughly 1.7x what Onkyo TX-RZ70 does. Expect it to lead on paper; the question is whether you will hear the difference through the speakers you own.
| Area | Better | Why |
|---|
| Power | evenly matched | Effectively the same output - 140W against 140W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Analogue inputs | B | Denon AVR-X6800H takes more analogue sources - 6 against 4. |
| Room to grow | evenly matched | The same expansion options on both. |
| Shelf space | depends | Denon AVR-X6800H 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 | A | Onkyo TX-RZ70 costs less - $2,300 against $3,799 at list. |
| Watts per dollar | A | Onkyo TX-RZ70 is the cheaper watt - about $16 per watt against $27. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Onkyo TX-RZ70 B Denon AVR-X6800H
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. Think about the whole system: the roughly $1,499 saved by taking the cheaper one would buy better speakers or a subwoofer, which does more for the sound than the difference between two competent amplifiers.
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.