Siblings in Klipsch's Reference Premiere (RP) series, so they share a family sound. It comes down to size, output and price.
| Area | Better | Why |
|---|
| Amplifier needs | evenly matched | Similar sensitivity (93dB vs 93dB) - both want much the same power. |
| Size | depends | Klipsch RP-400M is easier to place; Klipsch Reference Premiere RP 500SA is bigger, which usually goes with fuller bass. Your room decides. |
| Type | depends | A Passive Surround against a Passive Bookshelf - a different job rather than a better one. |
| Cabinet | depends | Sealed vs Ported. Sealed is tighter and happier near a wall; ported digs lower but wants space behind it. |
| Price | A | Klipsch Reference Premiere RP 500SA is cheaper - $299 against $399, about 25% less. |
A Klipsch Reference Premiere RP 500SA B Klipsch RP-400M
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?
Neither runs away with it - they trade advantages. Let the "depends" rows and your own room settle it. In a large or open-plan room, Klipsch Reference Premiere RP 500SA has the cabinet volume to fill it more easily; in a small room, near a wall or at desk distance, Klipsch RP-400M is the easier speaker to get right. Think about the whole system, not just the speakers: the roughly $100 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.