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 | B | Arcam SA45 has more power - 180W against 100W into 8 ohms. That is about 2.6dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Grip on hard loads | B | Arcam SA45 holds up better into a 4-ohm load: its output increases strongly to 300W, where the other increases strongly. That matters with demanding speakers far more than the 8-ohm figure does. |
| Noise floor | B | Arcam SA45 is the quieter design on paper - 110dB against 104dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | B | Arcam SA45 has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| 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 | A | Only Luxman L-505Z offers a pre-out/main-in loop - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Headphones | depends | Only Luxman L-505Z has a headphone output. Worth something if you listen late, nothing if you never do. |
| Amplifier class | depends | Luxman L-505Z is Class A/B and Arcam SA45 is Class G. Class A/B runs warm and is the long-established mainstream choice; 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. |
| Price | B | Arcam SA45 costs less - $5,500 against $6,995 at list. |
| Watts per dollar | B | Arcam SA45 is the cheaper watt - about $31 per watt against $70. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Luxman L-505Z B Arcam SA45
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?
Arcam SA45 is the easier recommendation: it leads on most of the measured areas and costs less. Take Luxman L-505Z if one of the "depends" rows above describes your system. If your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - Arcam SA45 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 - Arcam SA45 has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 100 watts is already more than most people ever use, and the extra 80 watts would be headroom you pay for and never hear.
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.