Hello. I recently purchased a powered Yamaha sub (YST-SW015) for my Kenwood VR-309 receiver. The receiver is a few years old, but has all the connections for surround sound, including a pre-out for the sub. However, in the Yamaha manual, it says if the receiver has less than two sets of pre-outs, don't plug it in there. I"m a bit confused by this, can anyone tell me why one pre-out would be a problem? Help is appreciated!