A surround against a bookshelf. They are built for different places in a room, so start with where these have to go rather than which looks better on paper.
| Area | Better | Why |
|---|
| Bass | B | Klipsch R-51M digs deeper - 62Hz against 94Hz. |
| Amplifier needs | B | Klipsch R-51M is more sensitive (93dB vs 89dB), so it goes louder on the same amplifier. |
| Size | depends | Polk Audio Monitor XT90 is easier to place; Klipsch R-51M 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. |
| Expandability | depends | Neither adds bi-amping, a sub output or height drivers. Polk Audio Monitor XT90 sits in a wider range (9 models), so a matched surround system is easier later. |
| Price | A | Polk Audio Monitor XT90 is cheaper - $119 against $359, about 67% less; arguably a different class. |
A Polk Audio Monitor XT90 B Klipsch R-51M
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?
The usual trade-off: Klipsch R-51M measures better, Polk Audio Monitor XT90 costs about about 3x the price% less. Whether that gap is worth the money is the call only you can make - and in a normal room it is often smaller than the numbers suggest. If a subwoofer is in your plans, the bass gap largely stops mattering - you would be crossing over above 94Hz anyway, which makes the cheaper Polk Audio Monitor XT90 much easier to justify. In a large or open-plan room, Klipsch R-51M has the cabinet volume to fill it more easily; in a small room, near a wall or at desk distance, Polk Audio Monitor XT90 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.