These sit in different price classes - KEF LSX II costs roughly 3.3x what Audioengine A5+ does. Expect it to lead on paper; the question is whether the gap is audible enough in your room to be worth it.
| Area | Better | Why |
|---|
| Bass | B | Audioengine A5+ digs deeper - 50Hz against 54Hz. A subwoofer would even that out. |
| Built-in power | A | KEF LSX II has the bigger amplifier (200W vs 50W RMS). |
| Connections | A | KEF LSX II adds Bluetooth, Wi-Fi, streaming, HDMI and digital inputs. |
| Size | depends | KEF LSX II is easier to place; Audioengine A5+ is bigger, which usually goes with fuller bass. Your room decides. |
| Expandability | depends | KEF LSX II: a sub output; Audioengine A5+: a sub output. KEF LSX II sits in a wider range (10 models), so a matched surround system is easier later. |
| Price | B | Audioengine A5+ is cheaper - $399 against $1,299, about 69% less; arguably a different class. |
A KEF LSX II B Audioengine A5+
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?
KEF LSX II is ahead, but only on 1 of the measured areas, and it costs about 3.3x the price. That is a lot of money for a narrow margin on paper - Audioengine A5+ looks like the sensible buy unless something else about the KEF LSX II appeals to you. In a large or open-plan room, Audioengine A5+ has the cabinet volume to fill it more easily; in a small room, near a wall or at desk distance, KEF LSX II is the easier speaker to get right. Think about the whole system, not just the speakers: the roughly $900 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.