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 | MartinLogan Illusion ESL C34A digs deeper - 45Hz against 54Hz. A subwoofer would even that out. |
| What you buy | depends | MartinLogan Illusion ESL C34A is a complete system - just add a source. ATC C4CA also needs an amplifier: more money and another box, but you choose it. |
| Connections | depends | With ATC C4CA that all comes from the amplifier you pair it with. |
| Upgrade path | depends | ATC C4CA can be re-amped later and outlive several amplifiers; MartinLogan Illusion ESL C34A ages as one unit. |
| Size | depends | ATC C4CA is easier to place; MartinLogan Illusion ESL C34A is bigger, which usually goes with fuller bass. Your room decides. |
| Cabinet | depends | Sealed vs Ported. Sealed is tighter and happier near a wall; ported digs lower but wants space behind it. |
| Price | evenly matched | Much the same money ($8,999 vs $9,500 a pair) - though the passive pair still needs an amplifier. |
A ATC C4CA B MartinLogan Illusion ESL C34A
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?
MartinLogan Illusion ESL C34A suits you if you want one box, ready to play, with nothing else to buy. ATC C4CA 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 54Hz anyway, so judge these on the other rows instead. In a large or open-plan room, MartinLogan Illusion ESL C34A has the cabinet volume to fill it more easily; in a small room, near a wall or at desk distance, ATC C4CA is the easier speaker to get right.
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.