Guillermo, most receivers can supply more than enough voltage on their sub output to turn on and drive the sub to a very high level. So, unless your receiver for some reason can't supply enough voltage even when, as you've said, you turn up the sub out, it would appear that the problem would be in the sub amp. The "knocking" solution would, of course, also tend to indicate this. Maybe you should speak with Joe again.

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Enjoy the music, not the equipment.