Don't you need balanced inputs, to do the subtraction?
I thought so, then I tried it with two splitters I had laying around and it worked for me in my home rig. Center information completely canceled...Don't you need balanced inputs, to do the subtraction?
Even my 1973 Pioneer SA-7100?I suspect your home rig might have balanced input circuitry, even if it's not in the specs.
If the inputs are balanced, then this is simple, you just invert the phase of one of the inputs. You can use a simple splitter to do that. If the amplifir doesn't have balanced inputs, you need to isolate the signal ground. A ground loop isolator will do. Invert the phase of one of the outputs (on the amplifier side of the ground loop isolator). You need one with transformers.