When you say you swapped cables at the pre-amplifier, do you mean to say you swapped cables at the output of the pre-amplifier to the amplifier or at the input to the phono stage? Because if you did the latter that is no different from swapping the cartridge pins except you have tested the cables themselves. My suspicion is that one channel is out on your pre-amplifier. The reason you have signal in mono is because you are hearing the right channel only but now it’s shared with the left. So, if you didn’t do it already, swap the cables that go from the preamplifier to the amplifier. Let us know what happens.
