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 | evenly matched | Effectively the same output - 150W against 180W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Grip on hard loads | A | Buchardt I150 holds up better into a 4-ohm load: its output nearly doubles to 300W, where the other barely increases. That matters with demanding speakers far more than the 8-ohm figure does. |
| Noise floor | A | Buchardt I150 is the quieter design on paper - 121dB against 95dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | A | Both decode on board. Buchardt I150 goes higher - 394kHz against 384kHz. Very few recordings exist above 192kHz, so treat this as headroom, not as sound quality. |
| Analogue inputs | B | NAD C 399 takes more analogue sources - 2 against 1. |
| Phono stage | depends | NAD C 399 has a built-in phono stage (MM) and Buchardt I150 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 | evenly matched | The same expansion options on both. |
| Headphones | depends | Only NAD C 399 has a headphone output. Worth something if you listen late, nothing if you never do. |
| Shelf space | depends | Buchardt I150 takes up about 37% less shelf area (1147cm² against 1818cm²), and it is 108mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | Buchardt I150 costs less - $2,500 against $2,899 at list. |
A Buchardt I150 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?
Buchardt I150 is the easier recommendation: it leads on most of the measured areas and costs less. Take NAD C 399 if one of the "depends" rows above describes your system. If your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - Buchardt I150 is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating. If you play records, NAD C 399 saves you buying a phono stage; if you do not, ignore that row entirely.
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.