Different kinds of product: one is active with its amplifier built in, the other passive and needs one. Judge them on how you want to own the system, not just the specs.
| Area | Better | Why |
|---|
| Bass | A | Buchardt A500 digs deeper - 25Hz against 33Hz. A subwoofer would even that out. |
| What you buy | depends | Buchardt A500 is a complete system - just add a source. Buchardt S400 MKII also needs an amplifier: more money and another box, but you choose it. |
| Connections | depends | Buchardt A500 has Wi-Fi built in. With Buchardt S400 MKII that all comes from the amplifier you pair it with. |
| Upgrade path | depends | Buchardt S400 MKII can be re-amped later and outlive several amplifiers; Buchardt A500 ages as one unit. |
| Size | depends | Similar cabinet size - neither is easier to fit. |
| Cabinet | depends | Sealed vs Passive Radiator. Sealed is tighter and happier near a wall; ported digs lower but wants space behind it. |
| Price | B | Buchardt S400 MKII is cheaper - $2,200 against $3,850, about 43% less; arguably a different class. Speakers only - an amplifier narrows that gap. |
A Buchardt A500 B Buchardt S400 MKII
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 speakers are judged; anything missing on either side is left out rather than guessed.
Which should you choose?
Buchardt A500 suits you if you want one box, ready to play, with nothing else to buy. Buchardt S400 MKII suits you if you already have an amplifier, want to choose the electronics, or expect to upgrade in stages. Neither is the better speaker in the abstract. If a subwoofer is in your plans, the bass gap largely stops mattering - you would be crossing over above 33Hz anyway, which makes the cheaper Buchardt S400 MKII much easier to justify. Think about the whole system, not just the speakers: the roughly $1,650 saved by taking the cheaper pair would buy a subwoofer or a better amplifier, and either usually does more for the sound than the difference between these two.
And remember what a spec sheet cannot show: how each looks in your room and whether the finish suits it, what dealer support and warranty are like where you live, and how they actually sound to you. If you can hear them first - ideally with your own music - that will tell you more than any number above.