Different price classes - Onkyo TX-RZ30 costs roughly 2.2x what Sony STR-DH590 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 - 90W against 100W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Speaker load rating | B | Onkyo TX-RZ30 is rated down to 4 ohms against 6, so it is the safer partner for a low-impedance speaker. |
| Analogue inputs | B | Onkyo TX-RZ30 takes more analogue sources - 6 against 4. |
| Phono stage | depends | Onkyo TX-RZ30 has a built-in phono stage and Sony STR-DH590 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 | B | Only Onkyo TX-RZ30 offers preamp outputs - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Shelf space | depends | Sony STR-DH590 takes up about 25% less shelf area (1318cm² against 1753cm²), and it is 95mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | Sony STR-DH590 costs less - $498 against $1,100 at list. |
| Watts per dollar | A | Sony STR-DH590 is the cheaper watt - about $6 per watt against $11. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Sony STR-DH590 B Onkyo TX-RZ30
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: Onkyo TX-RZ30 measures better, Sony STR-DH590 costs about 55% less. With most speakers in most rooms that gap is smaller in practice than it looks here. If you play records, Onkyo TX-RZ30 saves you buying a phono stage; if you do not, ignore that row entirely. Think about the whole system: the roughly $602 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.