Quartet15 extends PureAudioProject's open-baffle architecture vertically. Four 15-inch woofers sit in each channel, surrounding a central mid/high-frequency module. With no closed rear cabinet, the system operates as a dipole and radiates into the room in both directions.
The centre module is interchangeable, allowing different ribbon- or horn-based approaches to share the same bass frame. That makes Quartet15 closer to a configurable loudspeaker platform than a fixed finished box, with the final balance chosen around room, amplifier and listener preference.
Four large woofers are intended to provide radiating area, sensitivity and displacement headroom, not bass extension alone. Open-baffle behaviour also makes distance from the front wall and room modes decisive. A useful audition therefore needs to include the physical placement the owner can actually provide.