Perhaps your TDA3116D2 board is making an overload on the input channels. You should check the input impedances on the TDA3116D2 board. Generally in analog audio the signal output impedances are low, like 100 Ohms, and input impedances are high, like 47KOhms. This allows a good voltage transfer from output to input. If you lose half voltage when you load the output, that means the input impedance is equal to the output impedance, that should not happen. Input impedances must be high, not low.
The first input stage in your TDA3116D2 board is the lowpass filter. I guess in that stage also the summing of L and R signals is performed. Most surely the problem is there. Did you design and construct that LPF+summer? Summing of LR should not be passive, because the two channels would be mixed up, and you lose the stereo.
You should check with the oscilloscope what is hapenning and hunt down the problem. We don't know what really the problem is, consecuently we don't know the solution. Anyway, to put a buffers or active splitter in between could help. You could build up the buffers with a typical Opamp like TL072, with voltage follower setups.
On a second thought. All of these board are single supply (only +12V). Perhaps your LPF in the bass board is adding DC voltage component to the input signals, and then the signals are saturated in the other boards (there must not be DC component in signals). If you have an oscilloscope you should check that.