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 | Klipsch The Nines II digs deeper - 31Hz against 44Hz. |
| What you buy | depends | Klipsch The Nines II is a complete system - just add a source. Klipsch RP-600M II also needs an amplifier: more money and another box, but you choose it. |
| Connections | depends | Klipsch The Nines II has Bluetooth, HDMI and phono built in. With Klipsch RP-600M II that all comes from the amplifier you pair it with. |
| Upgrade path | depends | Klipsch RP-600M II can be re-amped later and outlive several amplifiers; Klipsch The Nines II ages as one unit. |
| Size | depends | Klipsch RP-600M II is easier to place; Klipsch The Nines II is bigger, which usually goes with fuller bass. Your room decides. |
| Expandability | depends | Klipsch The Nines II: a sub output; Klipsch RP-600M II: bi-amping. |
| Price | B | Klipsch RP-600M II is cheaper - $749 against $2,399, about 69% less; arguably a different class. Speakers only - an amplifier narrows that gap. |
A Klipsch The Nines II B Klipsch RP-600M II
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?
Klipsch The Nines II suits you if you want one box, ready to play, with nothing else to buy. Klipsch RP-600M II 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 44Hz anyway, which makes the cheaper Klipsch RP-600M II much easier to justify. In a large or open-plan room, Klipsch The Nines II has the cabinet volume to fill it more easily; in a small room, near a wall or at desk distance, Klipsch RP-600M II 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.