I don't think lack of line filtering would have anything to do with the problem. I would focus on the following possibilities:
-- Interconnect cables that might have an open shield connection somewhere, or that are excessively long, or that are low quality (specifically, that have excessive resistance across the length of the shield).
-- Oxidized plugs or jacks. Use contact cleaner if you have it, or if not just remove and reinsert the plugs a couple of times; the scraping action will often help.
-- Interconnect cable plugs that fit loosely, so that the ground connection is not being made securely.
-- Try plugging the ac power cords from all of the components into the SAME power strip or surge suppressor or outlet receptacle, if that is not already the case.
-- If any of the components have two-prong non-polarized power plugs, try reversing their polarity.
Hope that helps,
-- Al
-- Interconnect cables that might have an open shield connection somewhere, or that are excessively long, or that are low quality (specifically, that have excessive resistance across the length of the shield).
-- Oxidized plugs or jacks. Use contact cleaner if you have it, or if not just remove and reinsert the plugs a couple of times; the scraping action will often help.
-- Interconnect cable plugs that fit loosely, so that the ground connection is not being made securely.
-- Try plugging the ac power cords from all of the components into the SAME power strip or surge suppressor or outlet receptacle, if that is not already the case.
-- If any of the components have two-prong non-polarized power plugs, try reversing their polarity.
Hope that helps,
-- Al