Preamp line-out. I'm not certain, but sometimes these class D amplifiers act as the D2A as well as the amplifier, so there is no internal low level analog signal to tap off of. I think a simple 22:1 resistor divider from the speaker terminals would do it for you. You may have to combine the divider outputs from both the midrange and the tweeter. Likely the response will have been tailored for these specific drivers by the digital crossover, but that's probably OK. I would come off the positive terminal of each driver with a 2.2k resistor. Connect the other end of each resistor together and then to a 100 ohm resistor to ground. The line level will be across the 100 ohm resistor.