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 | B | Klipsch R-51M digs deeper - 62Hz against 65Hz. A subwoofer would even that out. |
| What you buy | depends | Mackie CR4-XBT is a complete system - just add a source. Klipsch R-51M also needs an amplifier: more money and another box, but you choose it. |
| Connections | depends | Mackie CR4-XBT has Bluetooth built in. With Klipsch R-51M that all comes from the amplifier you pair it with. |
| Upgrade path | depends | Klipsch R-51M can be re-amped later and outlive several amplifiers; Mackie CR4-XBT ages as one unit. |
| Size | depends | Mackie CR4-XBT is easier to place; Klipsch R-51M is bigger, which usually goes with fuller bass. Your room decides. |
| Expandability | depends | Neither adds bi-amping, a sub output or height drivers. Klipsch R-51M sits in a wider range (5 models), so a matched surround system is easier later. |
| Price | A | Mackie CR4-XBT is cheaper - $170 against $359, about 53% less; arguably a different class. And that is the whole system. |
A Mackie CR4-XBT 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?
Mackie CR4-XBT suits you if you want one box, ready to play, with nothing else to buy. Klipsch R-51M 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. 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, Mackie CR4-XBT is the easier speaker to get right. Think about the whole system, not just the speakers: the roughly $189 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.