Go into Save Song with Patches (ALT F2) and make sure the 'Save all settings with songs' is checked when you save. If not, the default style volumes will be used - usually 90.

Is the bass problem Midi or RealTrack? If Midi, does it happen on each playback? If not, you can freeze the bass track when it is correct. Try a different style and see if the problem persists. If it is a RealTrack, that is a different issue completely.

Rachael