Different price classes - PS Audio PMG S400 costs roughly 5x what PS Audio Stellar S300 does. Expect it to lead on paper; the question is whether you will hear the difference through the speakers you own.
| Area | Better | Why |
|---|
| Power | A | PS Audio PMG S400 has more power - 200W against 140W into 8 ohms. That is about 1.5dB, which is a small step in loudness but real headroom on peaks. |
| Room size | depends | We rate PS Audio PMG S400 the better fit for a very large room, and PS Audio Stellar S300 for desk or near-field listening, a small room and a mid-size room. Match the amplifier to the room you actually have, not the one you want. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output nearly doubles (400W and 300W). |
| Analogue inputs | evenly matched | The same number of line-level inputs (1). |
| Amplifier class | depends | PS Audio PMG S400 is Class A/AB and PS Audio Stellar S300 is Class D. 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 | PS Audio Stellar S300 takes up about 27% less shelf area (1316cm² against 1806cm²), and it is 102mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | PS Audio Stellar S300 costs less - $1,999 against $9,999 at list. |
| Watts per dollar | B | PS Audio Stellar S300 is the cheaper watt - about $14 per watt against $50. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A PS Audio PMG S400 B PS Audio Stellar S300
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?
Neither runs away with it - they trade advantages. Let the "depends" rows and the speakers you already own settle it. Think about the whole system: the roughly $8,000 saved by taking the cheaper one would buy better speakers or a subwoofer, which does more for the sound than the difference between two competent amplifiers.
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.