A valve amplifier against a solid-state one. They are not really rivals: they differ in character, in what they will drive and in what owning them involves, so start there rather than with the watts.
| Area | Better | Why |
|---|
| Power | A | PS Audio BHK Mono 300 has more power - 300W against 200W into 8 ohms. That is about 1.8dB, which is a small step in loudness but real headroom on peaks. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output nearly doubles (600W and 400W). |
| Analogue inputs | evenly matched | The same number of line-level inputs (1). |
| Amplifier type | depends | PS Audio BHK Mono 300 is Hybrid and PS Audio PMG S400 is Solid State. Valve designs are usually warmer and softer at the limits and need their tubes replaced eventually; solid state is the lower-maintenance choice. This is a taste and ownership decision, not a quality one. |
| Shelf space | depends | PS Audio BHK Mono 300 takes up about 15% less shelf area (1544cm² against 1806cm²), and it is 51mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | PS Audio BHK Mono 300 costs less - $8,499 against $9,999 at list. |
| Watts per dollar | A | PS Audio BHK Mono 300 is the cheaper watt - about $28 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 BHK Mono 300 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?
PS Audio BHK Mono 300 is the easier recommendation: it leads on most of the measured areas and costs less. Take PS Audio PMG S400 if one of the "depends" rows above describes your system.
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.