Chris, This is an interesting question and how this can be answered is first knowing what your use case is an what level of SPL you are trying to achieve. For example, if you have a background music application using distributed audio based on 70 V distribution then the Volume Controls could be from 10 to 35 W or more. Using your example with parallel wiring for each speaker (assuming 8 ohm spkr with a stepped transfomer) and a 100 Watt amp with 70v output the calculation would be: Zt = Z1 + N Zt = total impedance of speaker system  Z1 = the impedance of one speaker N = Number of speakers in the distributed system wired in parallel. From what I could find there is a very small insertion loss of only .5 dB on the Volume Control that uses a stepped tranformer design. So, I would not be too concerned about a insertion value for the Volume Control however the rating for wattage needs consideration.

So, the headroom on the amplifier should be selected on the basis of having at least 2-4 times the Continuous Power rating per channel. However this headroom need will depend on the music or voice application so the multiplier of output power would vary if the program is background music application all the way up to a rock band application. 

Below is a article that may help the overall understanding of distributed audio and amp selection.

I hope this helps... 

How Much | Crown Audio - Professional Power Amplifiers | English

Awesome Show! It was good to get back with a strong sense of networking and the bigger and better visual displays. 

Hi All, 

I just thought I would give this a spin. Xchange looks very promising for networking opportunities that are AV specific! Looks cool and fairly easy to move around and I've only been on this platform for a few minutes and I can find things!  Congratulations!