Hi all,
Years ago TOA had the sidearm of distributed PA impedence meters. I also have an Sennheiser and they are the right way to do things. If you look in Don Davis' Sound System Engineering (given to me by BD at Sound Equipment Corporation 31 years ago) there is a plan for a simple circuit a filiment transformer with a light bulb in series that causes speakers without the correct transformers to "climb out of the ceiling" an make themselves known. It is more durable and less expensive than using amplifiers.
Get some flavor of audio impedence meter and build one of these if you make your living on 70v and 25v systems.
Tom Maguire