These are not the same size of amplifier: WiiM Amp Ultra has roughly 2.5x the power of NAD C 328. That is about 4dB, 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 | A | WiiM Amp Ultra has more power - 100W against 40W into 8 ohms. That is about 4dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Grip on hard loads | A | WiiM Amp Ultra holds up better into a 4-ohm load: its output nearly doubles to 200W, where the other barely increases. That matters with demanding speakers far more than the 8-ohm figure does. |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Streaming | A | Only WiiM Amp Ultra has Chromecast and Roon. |
| Phono stage | depends | NAD C 328 has a built-in phono stage (MM) and WiiM Amp Ultra 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 328 has a headphone output. Worth something if you listen late, nothing if you never do. |
| Shelf space | depends | WiiM Amp Ultra takes up about 61% less shelf area (522cm² against 1349cm²), and it is 51mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | WiiM Amp Ultra costs less - $549 against $699 at list. |
| Watts per dollar | A | WiiM Amp Ultra is the cheaper watt - about $5 per watt against $17. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A WiiM Amp Ultra B NAD C 328
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?
WiiM Amp Ultra is the easier recommendation: it leads on most of the measured areas and costs less. Take NAD C 328 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 - WiiM Amp Ultra 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 - WiiM Amp Ultra has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 40 watts is already more than most people ever use, and the extra 60 watts would be headroom you pay for and never hear.
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.