If the preamp can make the sound smaller or bigger...but the preamp and amp are silent with no source attached to it no matter how loud you turn the preamp...then it has to be source. And is probably not grounding. i did not think grounding would get louder and softer with volume.
Might be your source or your cable in between your source and preamp. Try a different cable. If same, then try another dvd player. Hope the electrician can help.
Might be your source or your cable in between your source and preamp. Try a different cable. If same, then try another dvd player. Hope the electrician can help.