You can run the speaker outputs from the SEX to both your Orcas and the subwoofer. The speaker level inputs on the subwoofer are high impedance, so they won't put any additional load on the amp.
You can hook your Orcas to either the SEX amp or the output of the subwoofer. The subwoofer will usually have a basic high pass filter to relieve some of the low frequency content from traveling to your speakers.