These are not the same size of amplifier: JBL SA550 C has roughly 3x the power of Technics SU-GX70. That is about 4.8dB, 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 | JBL SA550 C has more power - 90W against 30W into 8 ohms. That is about 4.8dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Room size | depends | We rate JBL SA550 C the better fit for a mid-size room, a large room and a very large room, and Technics SU-GX70 for desk or near-field listening and a small room. Match the amplifier to the room you actually have, not the one you want. |
| Grip on hard loads | B | Technics SU-GX70 holds up better into a 4-ohm load: its output nearly doubles to 60W, where the other increases strongly. That matters with demanding speakers far more than the 8-ohm figure does. |
| Built-in DAC | B | Both decode on board, but only Technics SU-GX70 handles DSD. |
| Streaming | B | Only Technics SU-GX70 has AirPlay 2 and Chromecast. |
| Analogue inputs | A | JBL SA550 C takes more analogue sources - 3 against 2. |
| Use with a TV | depends | Only Technics SU-GX70 has HDMI ARC, 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. |
| Amplifier class | depends | JBL SA550 C is Class G and Technics SU-GX70 is Class D. Class G/H switches its supply rails to cut the heat a Class A/B design wastes; Class D runs cool and draws almost nothing at idle. Modern examples of both measure well - judge by the rest of the page. |
| Price | evenly matched | Priced within a few per cent of each other. |
| Watts per dollar | A | JBL SA550 C is the cheaper watt - about $22 per watt against $67. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A JBL SA550 C B Technics SU-GX70
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 - Technics SU-GX70 is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating. 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 - JBL SA550 C has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 30 watts is already more than most people ever use, and the extra 60 watts would be headroom you pay for and never hear - we rate JBL SA550 C poorly for near-field use for exactly that reason.
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.