These are not the same size of amplifier: PS Audio PMG M800 has roughly 2x the power of PS Audio PMG S400. That is about 3dB, 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 | PS Audio PMG M800 has more power - 400W against 200W into 8 ohms. That is about 3dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output nearly doubles (800W and 400W). |
| Analogue inputs | evenly matched | The same number of line-level inputs (1). |
| Shelf space | depends | PS Audio PMG S400 takes up about 40% less shelf area (1806cm² against 3020cm²), and it is 273mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | PS Audio PMG S400 costs less - $9,999 against $14,999 at list. |
| Watts per dollar | A | PS Audio PMG M800 is the cheaper watt - about $37 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 M800 B PS Audio PMG S400
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: PS Audio PMG M800 measures better, PS Audio PMG S400 costs about 33% less. With most speakers in most rooms that gap is smaller in practice than it looks here. 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 - PS Audio PMG M800 has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 200 watts is already more than most people ever use, and the extra 200 watts would be headroom you pay for and never hear - we rate PS Audio PMG M800 poorly for near-field use for exactly that reason. Think about the whole system: the roughly $5,000 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.