Charlie,
I have had the same experience on a number of occasions fishing wrecks...A few years back when I fished out of Dover, we only located the wreck and then switched of sounder as we found exactley what you experienced.
I know Rob Thompson on Shogun always used to switch of the sounder once the wreck was located and the drift was set. but as soon as other boats joined the drift the catch rate went down, but as you say combi units are a problem as I can't isolate my sounder.
Tomo