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 | Rotel Diamond RA-6000 has more power - 235W against 125W into 8 ohms. That is about 2.7dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Room size | depends | We rate MOON 330A the better fit for desk or near-field listening, a small room and a mid-size room, and Rotel Diamond RA-6000 for a very large room. Match the amplifier to the room you actually have, not the one you want. |
| Grip on hard loads | A | MOON 330A holds up better into a 4-ohm load: its output nearly doubles to 250W, where the other increases modestly. That matters with demanding speakers far more than the 8-ohm figure does. |
| Noise floor | B | Rotel Diamond RA-6000 is the quieter design on paper - 103dB against 100dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | B | Rotel Diamond RA-6000 has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Analogue inputs | B | Rotel Diamond RA-6000 takes more analogue sources - 4 against 2. |
| Phono stage | depends | Rotel Diamond RA-6000 has a built-in phono stage (MM) and MOON 330A 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. |
| Headphones | depends | Only Rotel Diamond RA-6000 has a headphone output. Worth something if you listen late, nothing if you never do. |
| Shelf space | depends | MOON 330A takes up about 23% less shelf area (1527cm² against 1979cm²), and it is 100mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | evenly matched | Priced within a few per cent of each other. |
| Watts per dollar | B | Rotel Diamond RA-6000 is the cheaper watt - about $19 per watt against $35. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A MOON 330A B Rotel Diamond RA-6000
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?
Rotel Diamond RA-6000 has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. If your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - MOON 330A is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating. Start with the room. In a large or open-plan room, or with speakers that are hard to drive - low sensitivity, or an impedance that dips - Rotel Diamond RA-6000 has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 125 watts is already more than most people ever use, and the extra 110 watts would be headroom you pay for and never hear - we rate Rotel Diamond RA-6000 poorly for near-field use for exactly that reason.
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.