Craig -
I'm like you; When I worked for a systems contractor, we dealt with 25 volt lines for schools, healthcare, 70.7 volt lines for other stuff, and even 200 volt line for large industrial facilities, where there was a speaker tapped for the wattage of the device.
I just stumbled across the same type of thing you describe while working on a system at a local church. Different brands, but same concept, where the transformer is in the volume control (or two for stereo operation), and there's a ratio setting that basically mulitiplies the speaker impedance by a factor of 1, 2, 4, or 8. It's a different way of thinking about it, requiring you to "do the math" using Ohm's law to get to the total load and ensure it doesn't drop below the amplifiers minimum rating.
There's a good tech sheet on Atlas-Soundolier's version of this approach here:
http://www.atlassound.com/support/SLSheets/SL4.1570.PDFGood luck!