So I'm on site now. 26 speakers. No volume controls that I can find. Checked voltage and amperage on the 70 and the 25. Volume turned all the way down on the amp, 1 kHz test tone. 70: 24.8 v 1.18 amps for 29.3 watts. 25v: 10.5 volts .4 amps for 22 watts.
Still waiting for the amp to heat up to see if the problem occurs again.
Using a multimeter to check those values won't tell you anything really. The 70V and 25V are RMS values and even if you have a true RMS multimeter it won't provide the correct reading (multimeters are designed to provide the correct RMS reading only at 60hz, although some of the cheap ones do give a close reading). Without an impedance meter, the next best this to use is an O'scope and calculate the RMS value.
The only way the values are going to be correct is when you input the test signal at the correct level and then take your reading. You'll never see an actual 70V or 25V signal on the lines as an example the last system that I worked on had a changing voltage reading around 4V.
Having 26 speakers can mean you are using 26 watts or up to 360 watts. It's the tap setting that you need to figure out if you don't have an impedance meter.
Here are a couple of good sources that will give you a better understanding on how this works.
Guide to constant-voltage systems A Handy Tool for Measuring “Constant Voltage” Sound System Impedance and Calculated Power doing a google search for 'testing a 70v system using a multimeter' will provide you with a wealth of information.
CAUTION - As with all google searches there is a lot more junk than useful stuff so be sure to cross-reference what you find with other sources
