Tarquinh:
Sorry, with plasterboard you should really only put load-bearing fixings into the metal or wood frames support the plasterboard. Anywhere else you risk an ugly hole in the wall, not to mention some speakers that may not have bounced that well!
Agreed - but with some care a secure fixture can be obtained.
I was thinking about a U shaped bracket (with the bottom of the "U" mounted against the wall) - you are primarily dealing with lateral loading here - so as long as nobidy tries to swing tarzan style from the speakers - should be fine. Of course it also depends on the depth of the chosen speakers too.
I have previously successfully attached some shelving which held quite a weight using those vertical supports, to which you clip in shelf brackets. Not the wieght of a speaker I do appreciate that.
My suggestion was only a though