These are not the same size of amplifier: NAD C 399 has roughly 6x the power of Technics SU-GX70. That is about 7.8dB, 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 30W into 8 ohms. That is about 7.8dB - a different class of amplifier rather than a bigger version of the same one, and far more than most rooms will ever ask for. |
| Room size | depends | We rate Technics SU-GX70 the better fit for desk or near-field listening, a small room and a mid-size room, and NAD C 399 for a large room and a very large room. Match the amplifier to the room you actually have, not the one you want. |
| Grip on hard loads | A | Technics SU-GX70 holds up better into a 4-ohm load: its output nearly doubles to 60W, where the other barely increases. That matters with demanding speakers far more than the 8-ohm figure does. |
| Built-in DAC | B | Both decode on board. NAD C 399 goes higher - 384kHz against 192kHz. Very few recordings exist above 192kHz, so treat this as headroom, not as sound quality. |
| Streaming | A | Only Technics SU-GX70 has AirPlay 2 and Chromecast. |
| Analogue inputs | evenly matched | The same number of line-level inputs (2). |
| Room to grow | B | Only NAD C 399 offers a subwoofer output - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Price | evenly matched | Priced within a few per cent of each other. |
| Watts per dollar | B | NAD C 399 is the cheaper watt - about $11 per watt against $67. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Technics SU-GX70 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?
NAD C 399 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 - Technics SU-GX70 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, 30 watts is already more than most people ever use, and the extra 150 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.