Hello. I recently found an old Pioneer VSX-9900s receiver lying around in the garage. It's about 16-18 years old and I want to use it with my audio set up. It only does stereo sound but has preouts in the back for center, sub, front, left and surround speakers. However, accompanying each preout is a "power in" RCA plug. I'm wondering what the power-in rca plugs are for.