Siblings in Polk Audio's Monitor XT series, so they share a family sound. It comes down to size, output and price.
| Area | Better | Why |
|---|
| Bass | A | Polk XT70 digs deeper - 35Hz against 94Hz. |
| Amplifier needs | B | Polk Audio Monitor XT90 is more sensitive (89dB vs 86dB), so it goes louder on the same amplifier. |
| Size | depends | Polk Audio Monitor XT90 is easier to place; Polk XT70 is bigger, which usually goes with fuller bass. Your room decides. |
| Type | depends | A Passive Floor-standing against a Passive Surround - a different job rather than a better one. |
| Cabinet | depends | Passive Radiator vs Ported. |
| Expandability | A | Polk XT70: Atmos drivers. |
| Price | B | Polk Audio Monitor XT90 is cheaper - $119 against $698, about 83% less; arguably a different class. |
A Polk XT70 B Polk Audio Monitor XT90
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 XT70 is ahead, but only on 1 of the measured areas, and it costs about 5.9x the price. That is a lot of money for a narrow margin on paper - Polk Audio Monitor XT90 looks like the sensible buy unless something else about the Polk XT70 appeals to you. 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, Polk XT70 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.