Can you download a 60 Hz test tone and take some AC measurements inside the amplifier?
The first would be between ground and the center pin of each RCA jack. I recommend turning your source all the way up to make the voltage easy to measure.
Next, turn the level control on the SEX all the way up and measure all the lugs on the pot. Two of them should be 0V (ground) and the other four should be about the same.
If those all test well, we can move along to the next check.
Frankly, it is a very, very bad idea to put an impedance matching kit on a SEX kit that isn't operational. This is the exact reason that we don't have instructions to do this right away, as now we will have to perform some extra checks to see if this is just a switching issue or is some kind of operational issue in the amplifier.