This looks like it to me too, the only thing I could think is when you adjust the volume on your HU it sends some odd signal through the sub out.i found the problem.... "old jvc cd hu"
thanks you have been a big help i'll try that tommorow with a newer cp huit really sounds like a problem with the head unit, can you get another one, like a friend that has another one laying around, just hook up power, ground and the rcas so you would just be playing the subs (would keep from having to hook up all the extras) and see if it still does that?the first stage of troubleshooting is isolating the problem to the faulty part in the system.
this would isolate it to the HU being bad, or tell you that it's something else if it still has the problem.