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 | Yamaha NX-70A digs deeper - 50Hz against 79Hz. |
| What you buy | depends | Yamaha NX-70A is a complete system - just add a source. KEF LS50 also needs an amplifier: more money and another box, but you choose it. |
| Connections | depends | Yamaha NX-70A has Bluetooth, Wi-Fi, streaming and HDMI built in. With KEF LS50 that all comes from the amplifier you pair it with. |
| Upgrade path | depends | KEF LS50 can be re-amped later and outlive several amplifiers; Yamaha NX-70A ages as one unit. |
| Size | depends | Similar cabinet size - neither is easier to fit. |
| Expandability | depends | Yamaha NX-70A: a sub output. KEF LS50 sits in a wider range (10 models), so a matched surround system is easier later. |
| Price | B | KEF LS50 is cheaper - $1,300 against $3,000, about 57% less; arguably a different class. Speakers only - an amplifier narrows that gap. |
A Yamaha NX-70A B KEF LS50
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?
Yamaha NX-70A suits you if you want one box, ready to play, with nothing else to buy. KEF LS50 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 79Hz anyway, which makes the cheaper KEF LS50 much easier to justify. Think about the whole system, not just the speakers: the roughly $1,700 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.