Hi. I hope that I can be of some help to you.
Speaker impedance is quite simple to calculate. Basically, amplifiers are designed to deliver power to a minimum design load. Typically, single speakers or speaker systems will have an impedance of 8 ohms. The lower the impedance, the more power will be sent to the speakers. The problem is that power amplifiers are designed to deliver maximum power to a minimum load. If the load is too low, and many professional amplifiers can deliver high power to loads of 2 ohms or less, the amplifier will quickly clip and may be damaged.
The rule then, is to look at the minimum load that your amplifier is designed to see, and keep you speaker designs within those limits.
For speakers connected in series, you simply add the impedances. So if two 8 ohm speakers are connected in series, the load seen by the power amplifier output will be 16 ohms.
For speakers connected in parallel, use this formula:
1/Imp Total=1/Imp 1 + 1/Imp 2 + 1/Imp 3 ...etc.
So. If you have three 8 ohm speakers connected in parallel, the total impedance will be:
1/Imp=1/8 + 1/8 + 1/8
Imp = 8/3 = 2.66 ohms.
I am not a fan of using external volume controls unless you know exactly how they effect the load seen by the amplifier. You should be able to control levels of different zones at the amplifier. Anyway, that is your choice. I just wanted to explain how to calculate impedance for series and parallel connections.