These are not the same size of amplifier: Arcam SA45 has roughly 2x the power of JBL SA550 C. 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 | Arcam SA45 has more power - 180W against 90W 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 increases strongly (300W and 150W). |
| Noise floor | A | Arcam SA45 is the quieter design on paper - 110dB against 106dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Streaming | A | Only Arcam SA45 has AirPlay 2, Chromecast and Roon. |
| Use with a TV | depends | Only Arcam SA45 has HDMI, so the television can drive it directly and one remote controls the volume. If the system is for music only, this changes nothing. |
| Room to grow | evenly matched | The same expansion options on both. |
| Headphones | depends | Only JBL SA550 C has a headphone output. Worth something if you listen late, nothing if you never do. |
| Price | B | JBL SA550 C costs less - $1,999 against $5,500 at list. |
| Watts per dollar | B | JBL SA550 C is the cheaper watt - about $22 per watt against $31. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Arcam SA45 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?
The usual trade-off: Arcam SA45 measures better, JBL SA550 C costs about 64% 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 - Arcam SA45 has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 90 watts is already more than most people ever use, and the extra 90 watts would be headroom you pay for and never hear. If this sits under a television, Arcam SA45 is the one to buy - HDMI ARC means the TV remote controls the volume and there is no second box in the way.
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.