If I understand your question you want to disconnect the PA amp (I HOPE you want to disconnect the amp) and try to drive the speaker system with the speaker outputs of a receiver that is designed for 4 to 8 ohm speakers.
A 70 volt system is intended to be driven by an amplifier with a 70 volt output which is what that PA amp has. You can use the receiver but I doubt you will be able to get sufficient volume before it starts clipping and other nasty things. You also have the problem of combining the two output channels into mono. If there is a stereo/mono switch use it, but I haven't seen one in 30 years. I don't think there is any consumer amplifier that would take kindly to the outputs being tied together so if you are going to try it use only the left or the right, NOT both!
As Iwire says, the proper way to do this is to take a line level output from the receiver and input it to the PA amp.
You are still going to have the problem of combining the two channels into mono. Using a "Y" cable may actually cause damage to the output devices feeding the jacks. Proper way is to combine each channel through something like 1K resistors.
-Hal
