The pot wipers are connected to the cathodes and the output jacks (J3 and J4). Yes, the signal is being taken off the cathodes which are connected to the pot wipers and the outputs. The grids are connected to J1 and J2. Whatever signal arrives, it arrives at the grids, which are like the gate of a MOSFET. Very very high impedance grids (gates). The grids allow current to flow through the valves and out the cathodes (or in, depending on how you think). More current as the grids become more positive. Much like a j-fet, the tube has some current flowing at zero voltage from grid to cathode. When the grids become more positive, more current flows and the resulting current through the cathode resistors developes a more positive voltage at the cathode. Thus, the voltage at the cathodes follow the voltage at the grids. The voltages will be almost exactly the same if the cathode resistors have enough resistance, and the tube tends to keep zero difference from grid to cathode. As the cathode resistors are adjusted to less and less resistance, the tube does not allow enough current to keep the grid to cathode voltage zero. The grids become more positive than the cathodes and the resultant increased difference turns the tubes on more fully. Thus, you have a common emitter, voltage follower circuit with no collector resistors.