A powered subwoofer should have a line-level input on it -- if all that it has is speaker input/output for the left and right speakers, with no power cord or line input(s), then it sounds a lot like an unpowered sub. In this case, you will need to do exactly what you describe: speaker wires from the left and right main outputs of the Carver 705 to the inputs on the sub, then speaker wires from the outputs of the sub to the main speakers.

------------------
Gonk