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 | JBL Summit K2 digs deeper - 25Hz against 32Hz. A subwoofer would even that out. |
| What you buy | depends | ATC EL50 is a complete system - just add a source. JBL Summit K2 also needs an amplifier: more money and another box, but you choose it. |
| Connections | depends | With JBL Summit K2 that all comes from the amplifier you pair it with. |
| Upgrade path | depends | JBL Summit K2 can be re-amped later and outlive several amplifiers; ATC EL50 ages as one unit. |
| Size | depends | ATC EL50 is easier to place; JBL Summit K2 is bigger, which usually goes with fuller bass. Your room decides. |
| Cabinet | depends | Ported vs Sealed. Sealed is tighter and happier near a wall; ported digs lower but wants space behind it. |
| Expandability | A | JBL Summit K2: bi-amping. |
| Price | evenly matched | Much the same money ($99,990 vs $99,999 a pair) - though the passive pair still needs an amplifier. |
A JBL Summit K2 B ATC EL50
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 EL50 suits you if you want one box, ready to play, with nothing else to buy. JBL Summit K2 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 32Hz anyway, so judge these on the other rows instead. Match it to your room: we rate JBL Summit K2 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.