It sounds like the subwoofer does not have a built in amp and the old RCA supplied the powered signal to the sub. Your only option if you still want to use that sub is to by an external amp.
+1for the cost of a good amp you migt as well just get a new powered sub like this one here.
How much did you pay for it? There may be better AVRs in your price range. If you can get the Onkyo suggested above at anywhere near the same price JUMP on it. You will be happier in the long run.By the way.. could you also tell me how good is my receiver. Any ideas.. I got this in a very good deal, however wanted to know your expert opinions from the experts. So, guys please help here.... I have 20 more days to return back this product. So far, seems to be real good... but... wanted your opinions as well