Played around with the speakers as planned and got some fairly good results. The Anthem ARC system lets you select one particular speaker and plays a sweep frequency through until you tell it to stop. You can move the microphone around during the test of course. It continuously updates the graph of the frequency response in relation to the room.
The centre speaker was giving me most concern because of a wide dip at around 100Hz which the ARC system couldn't correct properly -

Of course you can't mov the centre speaker very much right or left but I found that by moving it 3 inches back it improved things a bit -

- plus the ARC has levelled off the area around 1000Hz now.
Now I get the idea that you frown heavily when you hear talk about electronic room correction systems - but it has allowed me to get the uncorrected response better (that's the red line).
Of course improving the room would be helpful too - unless I plonked the speakers in the wrong place !
The surrounds are much better too since being moved around.

Might take a trip to some builders merchants next week to see about rockwool and rockfon.
