6 Vpp, do you mean 3 V peak? The gain is set to 4.3 so 3 V in will give you 12 V out which is the clipping voltage.
Do you have 9.1 V over the zener diodes? Supply voltage for the opamps should be the zener voltage minus 0.7 V which is 6.4 V in your case. If you have PLUS 12 V at pin 7, something is wrong.
The volume pot can be connected at the input and the value 10-47k. 10 k is better.