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 | A | PS Audio Stellar S300 has more power - 140W against 90W into 8 ohms. That is about 1.9dB, which is a small step in loudness but real headroom on peaks. |
| Room size | depends | We rate PS Audio Stellar S300 the better fit for a large room and a very large room, and JBL SA550 C for a small room. Match the amplifier to the room you actually have, not the one you want. |
| Grip on hard loads | A | PS Audio Stellar S300 holds up better into a 4-ohm load: its output nearly doubles to 300W, where the other increases strongly. That matters with demanding speakers far more than the 8-ohm figure does. |
| Noise floor | B | JBL SA550 C is the quieter design on paper - 106dB against 100dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | B | JBL SA550 C has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Analogue inputs | B | JBL SA550 C takes more analogue sources - 3 against 1. |
| Phono stage | depends | JBL SA550 C has a built-in phono stage (MM) and PS Audio Stellar S300 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. |
| Headphones | depends | Only JBL SA550 C has a headphone output. Worth something if you listen late, nothing if you never do. |
| Amplifier class | depends | PS Audio Stellar S300 is Class D and JBL SA550 C is Class G. Class D runs cool and draws almost nothing at idle; Class G/H switches its supply rails to cut the heat a Class A/B design wastes. Modern examples of both measure well - judge by the rest of the page. |
| Shelf space | depends | PS Audio Stellar S300 takes up about 16% less shelf area (1316cm² against 1565cm²), and it is 42mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | evenly matched | Priced within a few per cent of each other. |
| Watts per dollar | A | PS Audio Stellar S300 is the cheaper watt - about $14 per watt against $22. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A PS Audio Stellar S300 B JBL SA550 C
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. If your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - PS Audio Stellar S300 is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating. If you play records, JBL SA550 C 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.