These are not the same size of amplifier: NAD C 399 has roughly 7.2x the power of Rotel DX-5. That is about 8.6dB, so it is a question of room size and how hard your speakers are to drive, not of one being the better amplifier.
| Area | Better | Why |
|---|
| Power | B | NAD C 399 has more power - 180W against 25W into 8 ohms. That is about 8.6dB - a different class of amplifier rather than a bigger version of the same one, and far more than most rooms will ever ask for. |
| Grip on hard loads | A | Rotel DX-5 holds up better into a 4-ohm load: its output increases modestly to 33W, where the other barely increases. That matters with demanding speakers far more than the 8-ohm figure does. |
| Noise floor | A | Rotel DX-5 is the quieter design on paper - 100dB against 95dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Streaming | evenly matched | Only Rotel DX-5 has Roon. Neither list is longer in a way that matters - check the one service you actually pay for. |
| Phono stage | depends | NAD C 399 has a built-in phono stage (MM) and Rotel DX-5 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 NAD C 399 offers preamp outputs - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Amplifier class | depends | Rotel DX-5 is Class A/B and NAD C 399 is Class D. Class A/B runs warm and is the long-established mainstream choice; Class D runs cool and draws almost nothing at idle. Modern examples of both measure well - judge by the rest of the page. |
| Shelf space | depends | Rotel DX-5 takes up about 66% less shelf area (610cm² against 1818cm²), and it is 135mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | Rotel DX-5 costs less - $1,699 against $1,999 at list. |
| Watts per dollar | B | NAD C 399 is the cheaper watt - about $11 per watt against $68. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Rotel DX-5 B NAD C 399
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: NAD C 399 measures better, Rotel DX-5 costs about 15% less. With most speakers in most rooms that gap is smaller in practice than it looks here. If your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - Rotel DX-5 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 - NAD C 399 has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 25 watts is already more than most people ever use, and the extra 155 watts would be headroom you pay for and never hear - we rate NAD C 399 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.