For no apparent reason the sound level suddenly dropped on my box, but only on SD channels. As a result levels on these are now some 10db below the levels on HD channels.
I've contacted Humad tech support, who couldn't see any obvious reason for the change, but have referred the problem to their engineers. It would be very useful to know if anyone else has encountered the same problem.

Have you tried using the volume control on the Humax remote? That only affects the SD channels (well it does on mine anyway). Though you'd think Humax support would have asked that themselves

Thanks for the prompt reply. You ae absolutely right - I never normally use the volume on the Humax remote, preferring to use that on my Harmony remote instead. The volume on the Humax must have been changed by mistake, so when I increased it to max the normal level was restored. Once again thanks for your help.

Be interesting to see if Humax ever come back with a solution and how long it takes them to figure it out