SKW770B is the Sub, and based on the instructions it did not appear that it had a cutoff included.
The Amp is a Sony STR-DH130. Just checked again, definitely no Sub output
Just looked at the specs: http://www.intl.onkyo.com/downloads/manuals/pdf/skw-770_manual_e.pdf, should not be a problem (impedance is quite low - input) to do it as some posters suggested previously: + from R-channel and + from L-channel and obviously screen / ground. If you experience bad sound ad small 10K Ω resistance in series with each channel.