Issues with volume - DirecTV

This situation applies to all my TV's . I hope I can explain this correctly.

Listening volume is good for all HD channels but all SD channels have no loudness at all. The set top box audio setting is set to surround even though I have no home theater reciever connected. If I change the setting to stereo I then have good volume for all SD channels and no volume in the HD channel range.
If it makes any difference I have U300 level TV service. Why can I not have acceptable volume loudness with both HD & SD channels?

This question, "Issues with volume," is about DirecTV channels, programming, equipment, and sports.