Thanks again folks for the comments

I connected another pair of speakers and these worked fine, but they
were using the binding posts.

I got a reply from the chap who sold the speakers and he confirmed that
they should work the way I expected. I'm going to try and borrow some
speakers using the speakon connectors next.

I've been very careful with levels so I don't think I've done anything
silly :-)

A few more things to try so I'll let you know how I get on.

Dave