Here's more detail about what happens, just to help avoid this sort of problem --

The signal goes into the amp input, probably through a resistor, then on to a transistor. When the amps are on, voltages are present on the transistor that make it look to the signal like a linear high impedance load -- like a resistor.

When the amps are turned off, the excitement begins. The power voltages inside the amp drop to zero, at very low impedance, so now the + side of the transistor looks like a ground to the audio coming in. Since the voltages in the amp are at zero, the transistor's current flow now only comes from the input, limited only by the input voltage and the input resistance. Basically, the transistor now looks like a resistor in series with a diode across your audio signal. This will definitely cause distortion!

This does not happen with all amplifiers. In my experience, most since the mid 90s do not give you this kind of problem. It happens when you are trying to give your customer a break by using his old two-channel receiver for the second zone amp.

I have solved this by building a little harness that places a 1K resistor between the CD output and EACH input. This way, the effect of the "shorted diode" is minimized. In some cases, you might have to go as high as 4.7K. Suddenly you are spending your time and money to solve a problem caused by saving your customer the price of a new amp. I now avoid this when I can.

The Tibutaries piece will definitely solve this, and should be included in the initial price, just in case.