Adding a bit to what TLS said, even if you do get the impedance issue ironed out, you’re still over-taxing the amplifier. The problem is that the available power is divided out to all those speakers. For instance, if the receiver puts out 100 watts per channel, with six speakers connected to each channel, that means each one is only getting 16 watts. That’s probably why you can’t get the volume levels you want. Your best bet would be to get an outboard amplifier for the extra zones and separate them from the zone where the TV is.
Regards,
Wayne A. Pflughaupt