This is not an apple loops issue. It's a Channel strip settings issue relating to the Hybrid Bass and Hybrid Morph plugin instruments that come with Garageband, and IMHO produce a very nice sound too!
Something odd happens when you install Logic 8 that moves or deletes these files... Try installing just the relevant content from Logic 7 or Garageband from disc.
If you have another computer with Garageband on you can quickly find the relevant files and put them on your current computer:
Drag and drop folder Hybrid Morph to "HD/Library/ApplicationSupport/Garageband/InstrumentLibrary/Sampler/Sampler Instruments".
Then do the same to by taking Hybrid Synthesizers and put it with