You need an active external crossover between your preamp and your sub amp. There are 3 sets of outputs on your preamp use the 2nd L & R output to feed the crossover.
One excellent device, that will accomplish what you want, and be very transparent to your mains, is the Dahlquist DQ-LP1. They can be hard to find, at times. The are well worth the effort to seek out though. Passive High Pass/active Lo Pass, adjustable level for the low end, so you can match the levels with your mains. The unit will sum your channels, for one subwoofer/amp out, and so on. They usually bring at least what they sold for new, or slightly more. If you look for one, be certain to get the manual with it. ie: (http://www.allegrosound.com/Dahlquist_DQ-LP1_AllegroSound.html) (http://app.audiogon.com/listings/equalizers-dahlquist-dq-lp1-crossover-continuously-variable-low-pass-filter-2013-07-23-miscellaneous-34223-englewood-fl)