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Tom,
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Doug is right on the money. It might be dependant on which model ZYX one has, but the Cardas Frequency Sweep LP (available at Acoustic Sound for less than $ 30.00), makes a nice improvement when used regularly in my system.
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I find that I can hear clear improvements if I use it every 3 weeks. The longer I wait to use, it the more improvement is discernable.
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I would definitely get the Cardas LP and see what it does in your system.
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Rgds,
Larry