The 40th anniversary Quatuor model has added a rear tweeter, which allows the speaker to radiate energy from the front and back of the transmitting point, just like a symphony orchestra does. With this increase in energy, which is initially lost off-axis from the listening point, the sound volume is better distributed in front of the listener. This makes it easier to place the speaker in a room and get a stable and deep sound image with great openness without distorting the original signal.