If your subwoofer has a full range input and a cross-over output, then you're OK. Your sub doesn't do this. Otherwise, get a cross-over and a second amp.
Right now, your plan will drop your impedence down to 2.4 Ohms and may cause your amp to become unstable. The pass-thru on your sub is meant to go to another sub, and the purpose of that is to drop the ohms load to load the amp up more.