Hi ssbib
Thanks for that. I'm surprised, therefore, that the issue came up so often in customer reviews, followed in one case by a note from Polk saying it was a 'feature' and 'sorry'! Perhaps not all CEC controls are identical - I do hear that some implementations can be a bit iffy.
However, I have now gone for the Yamaha YAS-207. Very happy with it, and CEC control seems fine. But it does have a mildly annoying habit of switching itself on again 1-3 hours after everything else is switched off. Seems to be a random time, and happens even when all other input devices are unplugged from the TV. Will have to play a bit more with things like soundbar and TV standby settings. Not a show-stopper, just costing me a few extra pennies on the electricity bill!