Jeff -
I spent 17 years with a large systems contractor here in St. Louis, and we did a lot of constant-voltage sound system installations - 25 volt systems for educational and healthcare, 70 volt systems for commercial/industrial/retail, and even 200 volt systems for large places like racetracks and airplane factories. That being said, I'd be happy to share my $ .02 with you. I'll apologize up front if I'm telling you things you already know.
In a constant-voltage system, the rated voltage is the maximum voltage that the amplifier can cleanly produce at the rated load. Bear in mind, "clean" audio for PA/background music probably is around 2-5% THD, and the dynamic range of this material isn't nearly as wide as concert audio or any type of live performance. If you're feeding this system with live music, you're going to want a good quality limiter feeding the input to ensure that you don't overdrive the system beyond the 25/70/200 volts that it can produce.
Headroom is achieved by tapping speakers (selecting the appropriate wattage secondary winding on the speaker) to provide a good level when nominal audio signals are played, and then ensuring that you never exceed the rated voltage on the primary sides of the speakers/amplifier output. In no case do you want to put more than the rated voltage into the primaries of the transformers; it may or may not hurt the speakers, but would most likely damage the transformers themselves. (I've seen many of them fried over the years.)
Any of today's commercial ampflifiers should be able to run the full rated load with no problem; there is no derating for transformer loss. The transformer just causes some of that power to be turned into heat instead of sound. Of course, it never hurts to run any amp below it's rated capacity.
One common problem with constant-voltage sound systems is when someone has improperly wired an L-Pad to control a local speaker. The most common error is that it's wired so that it puts a dead short on the secondary of the transformer, which reflects right back to the primary/amplifier output side. On some occasions, I've seen them improperly wired across the primary side itself. These can often be difficult to locate, as the users may change the setting before you come out to troubleshoot, so check them for proper operation and wiring.
Also, check for non-transformer coupled speakers that someone may have added to the system - they can be loads of fun.
If you're going to do much work with constant-voltage systems, I'd highly recommend you invest in a commercial Z-meter, which measures the impedance of the load and is calibrated directly in wattage ratings for ease of use. The "easy" math for 70.7 volt systems is to remember that power = voltage squared/impedance, and conveniently, 70.7 squared is 5000. So a 500 watt 70 volt amplifier doesn't want to see less than a 10 ohm load. The Z-meter will "see" through the transformers and show you the true load, helping to find those shorts, bad speakers, and miswired controls, as well as knowing the real load on the system.
I know the McMartin and Toa both used to make these; I've been out of that business for 15 years, so I'm sure there are others by now.
Hope this helps!