Some years ago I had a Threshold SA-4e. The input impedance is 50 K Ohms unbalanced and 600 Ohm balanced.
I had an excellent Magnan Signature XLR cable which by design has a very high impedance of 30 K Ohms. My local Threshold service simply removed one resistor per channel which changed the balanced input impedance to something like 50 K Ohms.
It was easy and without any problems because this resistor's role was simply to lower the input impedance to match a studio norm of 600 Ohms. Thus removing it did not affect the amplifiers basic design or performance.
My Magnan Signature worked excellent with the SA-4e after the modification.