Guillermo, Marc's question, on the contrary, is quite perceptive and crossed my mind when I read that Joe had actually sent you a second new amp. The odds of both having the same defect would appear to be extremely low. It's possible that the "knocking" cure was actually vibrating the connector slightly and reconnecting a break, rather than acting on the sub amp itself. When the amp returns, if the problem persists and you have or can borrow any sort of cable with the RCA plugs, give it a try. You could even temporarily use speaker wire to the high level inputs on the sub as a test.
