It may be due to having the sound set to multi-channel and the amp treating the HD as maximum volume but for SD the Humax volume has been set at less than maximum.
To get SD and HD to be about the same either change the Humax to output stereo, or turn down the volume on your amp (to avoid any damage by the next action!), turn up the volume on the Humax to maximum, then adjust the amp back up to the volume you like. From then on don’t change the volume using the Humax remote as it will only impact SD channels unless you have changed the Humax sound option to Stereo.