To clarify what mark said (just because for some reason I had a bit of trouble following his post, although I know what he meant) - The back panel (Plate Amp) of your (most likely active) subwoofer should have speaker wire binding posts. The kind you turn, and either place bare copper or a banana plug into.
Connect speaker wire directly from the back of your yamaha to the wharfdale "speaker level inputs", in both right and left as indicated. Make sure the black wire is in the negative on both ends and the red wire is in the positive on both ends. Now connect speaker wire from the back of your sub directly to the respective right/left speakers in the same manner.
Next you will play around with the crossover "frequency" to as close-to-80hz as you can. If your speakers can go lower than that cleanly, then you may go lower. If you speakers don't go that low at all, you'll need to raise the frequency until the blend is seamless, although this isn't ideal because sound that's supposed to be coming from the right or left directions can be noticible heard from the direction of the subwoofer (80hz and under, this isn't nearly a problem). As mark said, what this does is sends a high pass filter to the main speakers (IE anything in the signal 80hz or higher) and sends a low pass filter to the subwoofer (anything 80hz or lower) where the subwoofer deals with it.
I suggest using something like DVE or Avia audio test tones to set this up properly..even for stereo these disks can be very useful, although they're a lot more useful in setting up 5.1 or 7.1. - and you may want nto pick up a radio shack (or something better like Galaxy CM130) SPL meter to get this as seamless as possible, since these devices are a lot more able to determine at which frequencies a speaker rolls off compared to our own ears, so finding the crossover frequency that will work best becomes easier.
