I installed PAC SN-1 ground loop isolaters on my car audio system because of alternator whine. They worked but now I have no low end bass just mids and highs.
What can I do to rectify the problem?
The spec of the SN-1 is fine, it shouldn't affect LF at all, so either the SN-1 is faulty, or there's something odd about the installation. Is it the same over both channels? If so, that reduces the chances of the SN-1 being at fault.
You could try another SN-1, or another different isolator, as there's nothing I can see to suggest that putting in a ground isolator would affect frequency response audibly.