Siblings in Polk Audio's Reserve series, so they share a family sound. It comes down to size, output and price.
| Area | Better | Why |
|---|
| Bass | A | Polk R400 digs deeper - 48Hz against 70Hz. |
| Amplifier needs | A | Polk R400 is more sensitive (89dB vs 86dB), so it goes louder on the same amplifier. |
| Size | depends | Polk Audio Reserve R900 is easier to place; Polk R400 is bigger, which usually goes with fuller bass. Your room decides. |
| Type | depends | A Passive Center Channel against a Passive Surround - a different job rather than a better one. |
| Price | evenly matched | Much the same money ($649 vs $599 a pair). |
A Polk R400 B Polk Audio Reserve R900
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?
Polk R400 has the edge on paper, though the margin is not large. The rows marked "depends" may matter more to you than the score does. If a subwoofer is in your plans, the bass gap largely stops mattering - you would be crossing over above 70Hz anyway, so judge these on the other rows instead. In a large or open-plan room, Polk R400 has the cabinet volume to fill it more easily; in a small room, near a wall or at desk distance, Polk Audio Reserve R900 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.