ringing the changes:

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Legend along botttom of graph is distance (cm) from front baffle to wall behind plane of speakers

Graph shows the variation of measured output with location from the rear wall. The turquoise trace was the previous preferred position, the red trace the final position chosen. Note the hump in the middle of the plot is due to low-order room modes boosting levels - 74hz is 2nd order mode across room, 59Hz is 2nd order along room and I can't do much about them at all.

Also, the speakers are setup on the long wall of the room, over 1700mm from the side walls - moving the speakers sideways has no effect discernible with the basic measuring equipment used.