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 | evenly matched | Much the same reach (56Hz vs 57Hz). |
| What you buy | depends | PSB Alpha AM3 is a complete system - just add a source. PSB P3 also needs an amplifier: more money and another box, but you choose it. |
| Connections | depends | PSB Alpha AM3 has Bluetooth and phono built in. With PSB P3 that all comes from the amplifier you pair it with. |
| Upgrade path | depends | PSB P3 can be re-amped later and outlive several amplifiers; PSB Alpha AM3 ages as one unit. |
| Size | depends | Similar cabinet size - neither is easier to fit. |
| Price | B | PSB P3 is cheaper - $279 against $449, about 38% less; arguably a different class. Speakers only - an amplifier narrows that gap. |
A PSB Alpha AM3 B PSB P3
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?
PSB Alpha AM3 suits you if you want one box, ready to play, with nothing else to buy. PSB P3 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. Think about the whole system, not just the speakers: the roughly $170 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.