You're quite right frank. I had specified -L 0db. I had rerun the test with -L -3db and updated the
picture. The LFE channel is now equal in voume to the centre channel
To be honest I wasn't exactly sure why the -3db was necessary but I notice HeadAC3he does it automatically.
I've added another picture (8) which shows that the transitions are not perfectly smooth. I wonder if this is due to the Dolby Digital compression.