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 | B | Bowers & Wilkins 801 D4 digs deeper - 15Hz against 60Hz. |
| What you buy | depends | ATC SCM150ASLT is a complete system - just add a source. Bowers & Wilkins 801 D4 also needs an amplifier: more money and another box, but you choose it. |
| Connections | depends | With Bowers & Wilkins 801 D4 that all comes from the amplifier you pair it with. |
| Upgrade path | depends | Bowers & Wilkins 801 D4 can be re-amped later and outlive several amplifiers; ATC SCM150ASLT ages as one unit. |
| Size | depends | Similar cabinet size - neither is easier to fit. |
| Cabinet | depends | Sealed vs Ported. Sealed is tighter and happier near a wall; ported digs lower but wants space behind it. |
| Expandability | B | Bowers & Wilkins 801 D4: bi-amping. |
| Price | evenly matched | Much the same money ($45,999 vs $46,000 a pair) - though the passive pair still needs an amplifier. |
A ATC SCM150ASLT B Bowers & Wilkins 801 D4
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?
ATC SCM150ASLT suits you if you want one box, ready to play, with nothing else to buy. Bowers & Wilkins 801 D4 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 60Hz anyway, so judge these on the other rows instead. Match it to your room: we rate Bowers & Wilkins 801 D4 the better fit for a very large room; elsewhere the two are rated the same.
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.