No single best speaker here - they are built to different priorities, and the right one depends on your room, your sources and what you listen to.
| Area | Better | Why |
|---|
| Bass | A | Kanto YU6 digs deeper - 50Hz against 60Hz. |
| Built-in power | A | Kanto YU6 has the bigger amplifier (100W vs 60W RMS). |
| Connections | A | Kanto YU6 adds phono and digital inputs. |
| Size | depends | Audioengine HD4 is easier to place; Kanto YU6 is bigger, which usually goes with fuller bass. Your room decides. |
| Expandability | depends | Kanto YU6: a sub output; Audioengine HD4: a sub output. |
| Price | B | Audioengine HD4 is cheaper - $449 against $540, about 17% less. |
A Kanto YU6 B Audioengine HD4
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?
Kanto YU6 leads across most of the measured areas for a fairly modest premium, which makes it the easier pick if the budget stretches. Audioengine HD4 is the one to take if it does not. If a subwoofer is in your plans, the bass gap largely stops mattering - you would be crossing over above 60Hz anyway, which makes the cheaper Audioengine HD4 much easier to justify.
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.