I'm gonna guess that there is either a problem with the amp on the 500 ( note that the new 500 amps seem to need more voltage sent from the pre-out on the receiver) or the sub pre-out on the receiver has gone bad.

Just to throw this out there, since the new amp for the 500 has been released with a subsonic filter, customers seem to have had a pretty consistent issues with "low ouput". Maybe time to re-examine these amps through quality control?

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