In one of the menu pages, I forget which, there is a setting you can choose between 'Line Mode' and 'RF Mode', that applies to programs transmitted with Dolby audio.
One of these, I think it's 'RF Mode', compresses things into a smaller dynamic range, so the volume more nearly matches SD channels.
There's only 2 to go at anyway, so it's worth experimenting (and sometimes settings have been known to need 'going away from and coming back to' in order to take effect!)